■ 


ft.  - V - sc 


RESEARCH  LIBRARY 
THE  GETTY  RESEARCH  INSTITUTE 


JOHN  MOORE  ANDREAS  COLOR  CHEMISTRY  LIBRARY  FOUNDATION 


ERRATA  TO  BURNING  CLAY  WARES 


Page  8,  twelfth  line  from  the  bottom,  insert  “in”  after 
“build  up”  to  read  “build  up  in  water  etc.” 

Page  8,  fifth  line  from  bottom,  “some”  should  be  “come.” 

Page  14,  fourteenth  line  from  top  “potach”  should  be 
“potash.” 

Page  14,  eleventh  line  from  bottom,  insert  a comma  “(,)” 
after  “mass”  and  omit  the  following  “and”  to  read  “portion 
of  the  clay  mass,  the  fused  material  etc.” 

Page  15,  second  line  under  the  heading  of  “The  Chief 
Minerals  in  Clay,”  change  the  comma  “(,)”  after  feldspar”  to 
a semi-colon  “(;).” 

Page  18,  twenty-second  line  into  twenty-third  line,  “prod- 
uct” should  be  “pro-duct.” 

Page  22,  fourth  line  under  “Lime”  heading,  omit  “as’’  to 
read,  “serious  menace  in  clay  burning  operations.” 

Page  24,  twelfth  line  from  bottom,  “motely”  should  be 
“motley.” 

Page  30,  fifteenth  line  from  bottom.  “Fe202”  should  be 
“Fe203.” 

Page  56,  seventeenth  line  from  bottom,  “Besemer”  should 
be  “Bessemer.” 

Page  61,  seventh  line  from  top  should  read,  “carbon  mon- 
oxide and  nitrogen;  from  a deeper  bed  we  get  carbon.”  The 
semi-colon  and  the  words  following  it  replace  the  period  and 
the  words  “The  first  result  is  wasteful  in”. 

Page  61,  ninth  line  from  top,  put  space  between  “mon- 
oxide and”. 

Page  63,  twenty-second  line  from  top,  put  “s”  after  com- 
bination”, making  it  plural. 

Page  67,  fifth  line  under  head  “Specific  Heat”,  change  “of” 
to  “or”  to  read  “in  calories  or  B.  t.  u.  etc.” 

Page  69,  Heading  “Thermal  Capacities  of  Cases”  should 
read  “Thermal  Capacities  of  Gases.” 

Page  69,  in  the  table  of  “Thermal  Capacities  of  Gases,”  in 
the  eighth  line  of  figures  beginning  “1432”  the  number  in 
the  fourth  column  should  be  “784”  instead  of  “748”. 

Page  73,  fourth  line  from  top,  the  first  number  should  be 
“.8173”  instead  of  “.9173”. 


Page  76,  twenty-fourth  line  under  heading  “Kiln  Tem- 
peratures,” add  “(See  pages  69,  70  and  71).” 
to  a foot  note  as  follows:  “Does  not  include  excess  air 

moisture  = 126.7  B.  t.  u.” 

Page  76,  twenty-fifth  line,  after  this  put  a referring 

Page  77,  first  line  under  the  table,  “readily”  should  be 
“really  get.” 

Page  77,  second  line  from  bottom,  “giving”  should  be 
“given.” 

Page  79,  put  a “*”  at  the  end  of  the  21st  line  referring  to 
a footnote  as  follows:  “Note — Recent  practice  uses  pressure 
in  the  oil  tank.” 

Page  81,  eleventh  line  from  bottom,  “molecule”  should  read 
“molecules.” 

Page  82,  first  line,  “bases”  should  be  “gases.” 

Page  96,  second  line  under  heading  “Sulphur  in  the  Ash,” 
“19  per  cent.”  should  read  “10  per  cent.” 

Page  98,  at  the  end  of  first  paragraph  add,  “(See  page  69) 

Page  98,  ninth  line  from  bottom,  “45  H to  CH4”  should 
read  “45  H to  35  CH4” 

Page  99,  second  and  third  line  from  the  bottom  put  a 
x 

period  “(.)”  after  “ ” then  spaces  between  this  and  “x  = 

2.2843  nitrogen.”  76.5 

Page  101,  ninth  line  below  the  first  table,  “dtermined” 
should  be  “determined.” 

Page  101,  tenth  line  below  the  first  table  following  “coal” 
add  “(See  page  69)” 

Page  102,  second  line  from  top  “71”  should  be  “17.” 

Page  103,  sixth  line  below  Heading  “Kiln  Temperatures, 
etc.”  add  “(See  Page  100).” 

Page  103,  second  line  from  bottom  put  a period  “(.)”  after 
x 

“- ” and  put  “x  = .6071”  in  the  last  line  since  it  otherwise 

16 

cannot  be  spaced. 

Page  104,  fourth  line  put  a period  (decimal  point)  before 
“4480”  which  should  read  “.4480.” 

Page  104,  in  the  table  toward  the  bottom  of  the  page,  the 
Figure  heading  the  first  column  should  be  “2232”  instead  of 
“2230.”  The  fourth  figure  in  this  column  should  be  “1593” 
instead  of  “1577.”  The  last  figure  (total)  should  be  “6928” 
instead  of  “6912.”  In  the  second  column  of  figures  change 
“1786”  to  “1804”  and  the  last  total  from  “7806”  to  “7824.” 


Page  105,  fourth  line  from  top,  change  “2.3975”  to  “2.4194.” 
Page  105,  in  the  table  toward  the  center  of  the  page  make 
the  following  changes: 

First  column  of  figures,  fourth  line  change  “1923”  to 
“1940”;  sixth  line  change  “7421”  to  “7438”;  ninth  line 
change  “8376”  to  “8393.’ 

Second  column  of  figures,  fourth  line  change  “2158” 
to  “2177”;  sixth  line  change  “8193”  to  “8212”;  ninth  line 
change  “9237”  to  “9256.” 

Third  column  of  figures,  fourth  line  change  “2402”  to 
“2424”;  sixth  line  change  “8906”  to  “9008”;  ninth  line 
change  “10120”  to  “10142.’ 

Fourth  column  of  figures,  fourth  line  change  “2661”  to 
“2686”;  sixth  line  change  “9813”  to  “9838.” 

In  the  second  line  below  this  table  change  “8376”  to  “8393.” 
Page  106,  twelfth  line  “klin”  should  be  “kiln.” 

Page  113,  fourth  line  above  heading  “Velocity  Head” 
change  “W”  to  “w”  reading  “w  = weight  of  a cu.  ft.  of  air 
at  32°” 

Page  118,  second  line  in  table  near  the  center  of  the  page, 
“Great  Resistance”  should  be  “Grate  Resistance.” 

Page  119,  in  center  of  page  after  “—  25.8  feet”  add  “(See 
page  113)” 

Similarly  in  the  eighth  line  from  bottom  after  “18.62  feet 
per  second”  add  “(See  page  111)” 

Similarly  in  the  fifth  line  from  bottom  after  “=  1.6”  add 
“(See  page  118)” 

Page  120,  first  line  add  “to”  to  the  end  of  this  line  to  read 
“whereas  relative  to  air  at  62  degrees,  etc.” 

In  the  ninth  line  from  top  after  “=  .06H”  add  “(See  page 
114)” 

Page  123,  second  line  from  top  add  “(See  page  70).” 

Page  123,  eighteenth  line  change  “is”  to  “are”  and  in  the 
same  line  change  “compartment”  to  “compartments.” 

Page  123,  fifth  line  from  bottom,  change  “inch”  to  “inches” 
and  to  the  end  of  this  line  add  “(See  page  113)” 

Page  124,  third  line  after  “=  .4H”  add  “(See  page  118)” 
Page  124,  sixth  line  put  a decimal  point  “(.)”  before  “6.” 
Page  125,  fourteenth  line  from  bottom  “drift”  should  be 
“draft.” 

Page  129,  twentieth  line  from  bottom  “repuired”  should 
be  “required.” 

Page  168,  figure  52  is  upside  down. 

Page  169,  eleventh  line  from  bottom,  “oftner”  should  be 
“oftener.” 


Page  174,  twenty-sixtji  line  from  bottom,  “care”  should 
be  “cars”;  fourteenth  line  from  bottom,  “converer”  should 
be  “converor”;  eighth  line  from  bottom,  “conveyer”  should 
be  “conveyor.” 

Page  175,  tenth  line  from  top  change  “conveyer”  to  “con- 
veyor”; fourteenth  line  from  top,  change  “conveyer”  to  “con- 
veror”; twenty-second  line  from  top,  change  “unites”  to 
“units.” 

Page  235,  nineteenth  line  from  top  should  follow  the  six- 
teenth line  to  read  as  follows,  beginning  with  sixteenth  line: 
“drain  tile  are  set  in  alternate  benches,  but  the  tile  benches 
are  enclosed  with  the  usual  setting  of  bricks  on  the  heads” 
then  follows  the  seventeenth  line,  “to  preserve  the  continuity, 
etc.” 

Page  249,  fifteenth  line  from  top,  “Schnatolla”  should  be 
“Schmatolla.” 

Page  254,  fifth  line  from  bottom,  change  “compartmenet” 
to  “compartment.” 

Page  284,  third  line  from  bottom  change  “gast”  to  “gas.” 

Page  285,  twentieth  line  should  read,  “ — Assume  six  com-  ' 
partments — one  and  two”  instead  of  “one  had  two.” 

Page  286,  fourth  line  from  top,  the  first  word  should  be 
“heim’s”  instead  of  „heims.” 

Page  329,  seventh  line  from  top  change  “lecing”  to 
“lecting.” 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/burningclaywaresOOIove 


BURNING  CLAY  WARES 


By  ELLIS  LOVEJOY,  E.  M. 

Member  and  Ex-President 
American  Ceramic  Society. 

Member 

American  Institute  of  Mining  Engineers, 
National  Brick  Manufacturers’  Association. 


THIRD  EDITION 


T.  A.  RANDALL  & CO. 
Publishers 
Indianapolis,  Ind. 


PREFACE, 


In  telling  a story  one  must  have  an  audience  and  if  the 
story  is  worth  while  it  must  be  adapted  to  the  audience. 

A learned  professor  after  solving  a difficult  problem  ex- 
pressed pleasure  that  it  had  no  practical  application. 

A story  from  him  would  be  interesting  to  a few  brother 
scientists,  but  it  would  be  over  the  heads  of  us  common  mor- 
tals. The  author  of  the  following  series  has  no  such  high 
attainment,  however  much  he  might  wish  it,  but  in  his  forty 
years’  preparation  as  student,  clayworker,  and  engineer  in  the 
clay  and  refractory  industries,  he  has  come  across  many 
things  one  would  like  to  know.  In  writing  the  articles  his 
first  consideration  was  the  audience.  To  whom  shall  he  write? 

The  thought  occurred  to  him — Why  not  write  to  himself? 

These  are  the  things  he  would  liked  to  have  known  when 
as  a beginner  in  the  manufacture  of  clay  wares  he  was  blun- 
dering through  difficulty  after  difficulty.  How  big  they  looked 
to  him  before  they  were  overcome,  but  how  they  dwindled 
and  were  soon  forgotten  afterward.  As  the  years  went  by  he 
gained  confidence  with  experience,  but  new  problems  were 
coming  up  to  be  solved,  other  difficulties  to  be  overcome,  and 
he  is  now  telling  himself  how  he  could  have  solved  these  prob- 
lems and  overcome  the  difficulties,  or  how  futile  were  his 
efforts. 

As  an  engineer  in  the  clayworking  industries,  the  field  of 
his  experiences  greatly  widened,  and  many  interesting  prob- 
lems have  been  presented  to  him  for  solution. 

In  his  story  he  has  become  reminiscent,  with  himself  as  the 
listener. 

In  writing  the  story  he  came  upon  many  problems  which  he 
found  in  his  experience  he  had  only  partially  solved  and  he 
must  work  them  out  to  prepare  his  monthly  lecture  to  himself. 
The  work  has  been  interesting.  When  one  is  relating  his  ex- 
periences, it  is  always  interesting  to  him,  and  if  he  is  talking 
to  himself,  it  is  equally  interesting  to  the  one  who  listens. 

And  so  the  story  has  been  written  by  a clayworker  to  a 
clayworker,  and  if  other  clayworkers  find  something  of  inter- 
est, something  of  value,  in  the  series,  the  author  will  be  greatly 
pleased  and  rewarded  for  his  effort. 

A great  deal  more  could  have  been  said  and  some  may  wish 
that  it  had  been  included,  although  it  would  have  made  the 
series  unduly  long.  Others  may  find  the  series  all  too  long 
and  will  feel  that  much  could  have  been  omitted  without  any 
loss  to  the  industry.  Both  are  right,  but  the  author  is  not 
concerned  because  he  has  just  been  talking  to  himself. 

ELLIS  LOVEJOY,  E.  M. 


TABLE  OF  CONTENTS. 


Page 


CHAPTER  I 

Clays  and  Their  Mineral  Contents 

Kaolinite  

Fluxes  and  Eutectics  

Bond  

The  Chief  Minerals  in  Clay 

Kaolin  

Quartz  

Mica  

Lime  

Feldsar  

Carbon  

Iron  Materials  

Magnesia  

CHAPTER  II 

The  Burning  Process 

Watersmoking  

Oxidation  

Shrinkage  

Vitrification  

Flashing  

Cooling  

CHAPTER  III 

Burning  Behavior  of  Clays 

Color  Changes  

Behavior  of  Various  Types  of  Clays. . . . 

Salt  Glazing 

Causes  of  Blisters  

Pimples  or  Rough  Pipe 

Crazing  and  Cracking 

Bloating  and  Black  Coring 

CHAPTER  IV 

Fuel  and  Combustion  

The  Need  of  Better  Understanding 

Fuels  and  Their  Burning  Properties . . . 

Measure  of  Heat  Unit 

Heat  Losses  in  Kiln  Burning 

Evaporation  of  Water  

Unburned  Carbon  in  Ash 

Heat  of  Ash  

Incomplete  Combustion  and  Excess  Air 

Specific  Ileat  

Calorific  Determination  

Carbon-Dioxide  in  Combustion  Cases . . 

Radiation  Losses  

Kiln  Temperatures  

Advantages  of  Fuel  Oil 

Producer  Gas  

Producer  

Operation  of  Producer 

Carbon  in  Ash  


7 

8 

9 

14 

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28 

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60 

64 

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67 

69 

74 

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76 

77 

81 

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82 

95 


CHAPTER  V 

Page 

Sulphur  in  Ash  96 

Steam  for  Blast  96 

Evaporation  of  Moisture  97 

Tar  and  Soot 97 

Calorific  Value  101 

Kiln  Temperatures  from  Producer  Gas 106 

CHAPTER  VI 

Stacks  106 

Kiln  Stack  Problems  106 

Intensity  of  Draft 108 

Area  of  Stack  110 

Total  Draft  Intensity 112 

Velocity  Head  114 

Friction  Head  114 

Maximum  Efficiency  Temperature  115 

Stack  Heads  and  Weights  of  Gas  Moved 117 

Stack  Furnaces  117 

Height  of  Stack  for  Periodic  Kiln 118 

Continuous  Kiln  Stacks  123 

Construction  of  Stacks 126 

Induced  Draft  128 

CHAPTER  VII 

Furnaces  136 

Secondary  Air  137 

Coking  Table  Furnaces 139 

Pit  Furnaces  141 

Unclassified  Furnaces  143 

Position  and  Size  of  Furnace 145 

Comparison  of  Furnace  Areas 148 

Furnace  Doors  . 150 

Construction  of  Furnace 151 

CHAPTER  VIII 

Kilns  153 

Classification  of  Kilns  153 

Periodic  Open  Top  Kilns 154 

Kiln  Sheds  157 

Setting  157 

Furnaces  and  Burning  164 

Coaling  171 

Machine  Handling  and  Setting  173 

Open  Top  Continuous  Kilns. 176 

Periodic  Crowned  Updraft  Kilns 183 

Down-Draft  Periodic  Kilns  187 

Rectangular  Down-Draft  Kilns  187 

Multiple  Stack  Kilns  188-204 

Rectangular  Kilns  with  Outside  Stacks 190 

Features  in  the  Construction  of  Rectangular  Kilns 194 

Sand  Pockets  196 

Setting  197 

Round  Down-Draft  Kilns  199 

Types  of  Round  Kilns 201 

General  Construction  of  Kiln  Bottoms 201 

Center  Stack  Kilns  201 


Single  Outside  Stack  Kilns 

Banding  Round  Kilns  

Up  and  Down  Draft  Kilns 

Horizontal  Draft  Kilns  

Muffle  Kilns  

Muffle  Kiln  Construction  

CHAPTER  IX 

Some  Notes  on  Setting 

Setting  for  Flame  Effect 

Setting  Common  or  Face  Brick 

Setting  for  Flashed  Brick 

Setting  in  Bungs  for  Salt  Glazing 

Setting  Roofing  Tile 

Setting  for  Vitrified  Tile 

Setting  for  Spanish  Tile  and  Finials 

Setting  Terra  Cotta 

Setting  Sewer  Pipe  

Setting  Elbows,  Branches,  Tees,  Traps,  Etc 

Setting  Enameled  Brick 

Setting  Silica  Brick  

Setting  Magnesite  Brick  

Setting  Pottery,  Porcelain,  Abrasives,  Etc 

CHAPTER  X 

The  Continuous  Kiln 

Economizer  Kilns  in  General 

Kiln  Dampers  

Open  Top  Economizer  Kilns 

Ring  or  Tunnel  Kilns 

Zig  Zag  Kiln  

Compartment  Kilns  

Kiln  Arrangement  

Producer  Gas  Economizer  Kiln 

CHAPTER  XI 

Car  Tunnel  Kiln  

Advantages  and  Disadvantages  

Products  Successfully  Burned  

Early  Car  Tunnel  Kilns 

Drayton  Kiln  

Faugeron  Kiln  

Heat  Balances  of  Car  Tunnel  Kilns 

Dressier  Kiln  

Hoffman  Kiln  

Comparative  Fuel  Consumption 

Zwerman  Kiln  

Harrop  Kiln  

Other  Car  Tunnel  Kilns 

CHAPTER  XII 

Burning  a Down  Draft  Kiln 

Coloration,  Discoloration  and  Other  Burning  Effects 

Scum  

Fireflashing  

APPENDIX 

Equalization  Tables  

Tables  of  Rectangles  in  Equivalent  Circles 


Page 
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. 214 
. 216 
. 223 
. 225 
. 231 


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Copyrighted  1920-1922 

by 

T.  A.  EANDALL  & CO. 


BURNING  CLAY  WARES. 


7 


BURNING  CLAY  WARES 

By  ELLIS  LOVEJOY 

CHAPTER  I. 

CLAYS  AND  THEIR  MINERAL  CONTENTS. 

A CLAY  IS  NOT  a mineral  in  a strict  sense  of  the  term 
mineral  but  instead  is  a complex  mixture  of 
minerals. 

In  order  to  carry  out  the  burning  process  intelligently  we 
should  have  some  understanding  of  the  minerals  with  which 
we  are  dealing  and  their  behavior  under  fire. 

Each  clay  is  a separate  problem  because  it  differs  from 
other  clays  in  its  mineral  content.  The  same  mineral  in  dif- 
ferent clay  does  not  give  the  same  effect  in  each.  Iron,  for 
instance,  may  produce  a deep  red  color  even  through  a long 
vitrification  range;  it  may  develop  a deep  red  changing  to  a 
dark  brown;  the  color  may  be  a pale  red  or  even  a buff  at 
certain  temperatures  while  at  higher  temperatures  the  ware 
may  become  peppered  with  black  iron  spots;  associated  with 
lime  it  may  produce  a pale  red  at  low  temepratures,  a buff 
at  medium  temperatures,  and  a green  at  high  temperatures. 
The  amount  of  iron  may  be  the  same  in  each  case  and  the 
different  results  are  due  to  the  chemical  and  physical  char- 
acter of  the  iron  minerals  and  their  reaction  with  associated 
minerals. 

The  common  base  of  all  clays  is  kaolinite,  a hydrated  sili- 
cate of  alumina.  There  are  other  hydrated  silicates  which 
closely  resemble  kaolinite  in  their  physical  properties,  par- 
ticularly plasticity,  but  so  far  as  our  burning  problems  are 
concerned  we  need  only  consider  kaolinite. 


8 


BURNING  CLAY  WARES. 


Kaolinite  comes  from  the  disintegration  of  crystalline 
rocks,  especially  feldspar.  The  latter  is  a silicate  of  alumina 
with  some  base,  such  as  potash,  soda,  lime,  magnesia,  iron, 
etc.  In  the  disintegration  process,  silica  in  part  is  set  free, 
kaolinite  is  formed,  and  the  base  is  set  free  to  combine  with 
some  acid  radical  to  form  sulphates,  carbonates,  etc. 

Kaolinite  is  considered  a final  product  from  the  disintegro- 
tion  of  feldspar,  but  there  is  evidence  to  show  that  under 
conditions  not  now  understood,  kaolinite  is  disintegrated  to 
form  bauxite  (hydrated  alumina)  and  silica.  From  the  pure 
feldspar,  then,  we  have  several  derived  minerals  any  or  all 
of  which  may  be  found  in  the  clay  mass,  but  feldspar  is  only 
a part,  often  a very  small  part,  of  crystalline  rocks. 

It  is  associated  with  quartz,  mica,  hornblendes,  augite, 
and  a long  series  of  other  minerals,  usually  complex,  which 
disintegrate  to  form  other  derivative  minerals,  and  all  of 
these  minerals,  both  original  and  derived,  may  be  found  in 
the  clay  body. 

Finally,  as  the  resulting  minerals  are  washed  from  their 
source  to  their  final  resting  place,  they  are  not  only  under- 
going mineralogical  changes,  but  they  are  being  sifted  and 
sorted  by  moving  water  and  deposited  as  sands  and  clays,  or 
on  the  contrary,  perhaps,  they  are  being  mixed  with  the  waste 
from  other  sources  and  the  complexity  of  the  mass  increased 
to  develop  into  trouble  for  the  clayworker.  On  the  one  hand 
we  may  have  pure  kaolin  and  on  the  other  common  clay,  and 
between  there  will  be  all  gradations  from  the  one  to  the 
other. 

Nor  is  the  story  fully  told — it  is  too  long  to  tell  here — 
since  the  development  of  the  mineral  mass  we  call  clay  only 
begins  with  the  disintegration  of  the  rocks.  The  deposits 
may  gradually  build  up  water  teeming  with  life  the  re- 
mains of  which  go  into  the  clay  bed  to  be  reckoned  with 
later  in  our  kiln  problems;  they  may  subsequently  become 
land  surfaces  covered  with  vegetable  growth;  again  be  buried 
to  great  depths  and  hardened  by  heat  and  pressure,  the  vege- 
table matter  converted  into  lignite,  coal,  graphite,  and  the 
pores  of  the  clay  mass  filled  with  the  distillates  of  the  ani- 
mal and  vegetable  remains ; again  some  to  the  surface  by 
some  wrinkle  in  the  earth’s  crust,  be  eroded  and  removed  to 
other  locations. 

A flood  from  the  north  may  bring  one  bed,  superimposed 
by  a bed  left  by  a flood  from  the  east,  and  this  covered  by  a 


BURNING  CLAY  WARES. 


9 


flood  deposit  from  the  west,  each  bed  perhaps  widely  differ- 
ent. Not  only  will  the  beds  be  one  above  the  other  but  each 
succeeding  flood  may  cut  gullies  in  the  beds  already  depos- 
ited and  fill  them  with  its  material. 

The  clays  when  deeply  buried  and  subjected  to  pressure 
and  heat  are  converted  into  shales  but  without  losing  any  of 
their  properties;  under  greater  pressure  and  heat  the  shales 
become  schists  and  slate  and  lose  their  plasticity  which  is  of 
first  importance  to  the  clayworker.  This  property  is  largely 
irrevocably  lost  and  the  schists  where  exposed  crop  out  as 
massive  rocks.  Many  of  them,  however,  still  contain  the  rem- 
nants of  the  original  clay-forming  minerals  and  these  are 
being  decomposed  to  break  down  the  schists  and  give  to 
them  some  measure  of  plasticity  which  they  must  have  to 
be  useful  to  the  clayworker. 

Ground  waters  are  circulating  through  the  clay  beds  puri- 
fying some  and  defiling  others.  Iron  minerals  are  being  dis- 
integrated and  the  iron  converted  into  oxides,  sulphites,  sul- 
phates, and  carbonates;  lime,  magnesia  and  the  alkalies  are 
being  changed  into  sulphates  and  carbonates. 

Geologists  have  never  given  us  a satisfactory  theory  to 
explain  the  buff  burning  and  refractory  clays  found  in  the 
coal  measures. 

The  latest  theory  is  that  advanced  by  Mr.  Wilber  Stout, 
of  the  Ohio  Geological  Survey,  in  Yol.  XVII,  Transactions  of 
the  American  Ceramic  Society.  The  theory  in  brief  is  that 
coal  beds  have  completely  oxidized  and  the  remaining  ash 
hydrated  which  would  give  a clay  having  the  composition  of 
coal  ash. 

It  would  seem  as  if  everything  in  nature  leads  up  to  clay 
or  plays  a part  in  the  development  of  clay. 

The  clayworker’s  problem  is  to  take  these  clays,  un- 
limited in  their  variations,  and  burn  them  into  a limited 
number  of  wares.  He  is  dealing  with  materials  the  fusion 
temperature  of  which  vary  from  cone  010  to  cone  42. 

Fluxes  and  Eutectics. 

The  common  conception  of  a flux  is  that  of  an  easily  fusi- 
ble chemical  or  salt  which  melts  at  a low  temperature  and 
dissolves  the  refractory  materials  to  be  fused,  either  with  or 
without  chemical  reactions  although  chemical  combination 
frequently  takes  place  resulting  in  the  formation  of  a differ- 
ent compound  or  mineral. 


10 


BURNING  CLAY  WARES. 


We  have  considered  the  potash,  etc.,  in  minerals  as  the 
flux,  hut  the  study  of  solutions  has  developed  results  which 
compel  us  to  modify  our  original  conceptions.  It  is  the  min- 
eral that  is  the  flux  and  not  the  mineral  content  considered 
separately,  moreover,  the  fluxing  effect  is  due  to  an  intimate 
mixture  of  the  minerals  to  be  fused.  Feldspar,  mica,  etc., 
will  act  as  fluxes  at  the  fusing  temperature  of  these  minerals, 
but  they  will  do  more  than  this  if  finely  ground  and  intimately 
mixed  with  the  minerals  to  be  fluxed,  namely,  they  will  act 
as  fluxes  at  temperatures  below  their  fusion  point. 

As  has  been  shown  by  Seger  and  others,  if  we  mix  kaolin 
and  silica  we  develop  without  any  flux  a mixture  more  fusi- 
ble than  either  alone.  With  successive  additions  of  silica  to 
kaolin  the  fusion  temperature  drops  from  cone  35  to  about 
cone  26,  but  further  additions  increase  the  temperature  re- 
quirement. This  minimum  fusion  point  is  called  the  eutectic 
and  the  mixture  which  fuses  at  the  minimum  temperature  is 
the  eutectic  mixture. 

In  this  mixture  there  is  no  flux  as  the  term  commonly  im- 
plies, but  the  mixture  itself  acts  as  a flux.  In  any  mixture  of 
kaolin  and  silica  some  part  of  it  will  be  in  the  eutectic  pro- 
portion and  this  will  start  the  fusion. 

The  most  fusible  mixture  in  a clay  body,  regardless  of 
the  content  of  a fluxing  salt,  such  as  potash,  is  the  flux,  and 
while  this  mixture  may  contain  the  alkalies  it  does  not  nec- 
essarily contain  all  the  alkalies  in  the  clay  mass;  indeed,  it 
may  contain  a very  small  part  of  them. 

The  fluxes  in  a clay  body  are  not  potash,  soda,  lime,  iron, 
etc.,  but  instead  they  are  more  or  less  complex  minerals  and 
mixtures  of  minerals. 

The  bases  are  a part  of  these  minerals  and  enter  into  the 
mixtures,  and,  no  doubt,  aid  in  developing  more  fusible  eutec- 
tic mixtures,  but  as  constituents  of  minerals  which  do  not 
enter  into  the  fusible  mixture  they  play  no  part  in  the  fusion. 

Paving  brick  which  we  class  as  vitrified  often  come 
through  the  kiln  heavily  scummed  and  the  lime  in  the  scum 
though  an  active  flux  under  certain  conditions  had  no  part 
in  the  fusion  which  produced  the  vitrification. 

In  the  form  of  an  oxide  the  lime  undoubtedly  would  have 
been  a part  of  the  fused  matrix  but  as  a sulphate  it  remains 
free  to  appear  as  scum. 

Mica  is  often  seen  in  well-burned  bricks  and  its  fusible 


BURNING  CLAY  WARES. 


11 


base  has  not  entered  into  the  fused  matrix  which  makes  the 
bond. 

Potash,  soda,  and  magnesia  are  found  in  wall  efflorescence 
and  there  is  ample  evidence  to  show  that  they  come  from 
the  clay  ware.  They  are  fusible  at  very  low  temperatures 
and  we  often  wonder  how  they  could  have  escaped  the  fire 
influence.  It  is  likely  that  the  heat  has  affected  some  of  the 
minerals  which  did  not  enter  into  the  fusion  and  that  in 
consequence  they  disintegrate  under  weather  influence,  thus 
setting  free  the  base  to  develop  efflorescence,  and  it  is  also 
likely  that  some  of  the  fused  matrix  is  not  stable  and  sim- 
ilarly disintegrates.  The  leaching  out  of  these  bases  has  no 
appreciable  effect  on  the  bond  of  the  ware  and  if  it  has  none 
whatever,  which  remains  to  be  proven,  then  we  must  con- 
clude that  the  bases  in  the  efflorescence  come  from  minerals 
which  were  not  a part  of  the  fused  matrix.  A mineralogicai 
examination  of  vitrified  ware  will  undoubtedly  reveal  a num- 
ber of  minerals,  containing  alkalies,  which  have  not  become 
a part  of  the  fused  matrix. 

An  important  factor  in  fusible  mixtures  is  a fine  state  of 
division  and  intimate  association  of  the  minerals. 

Two  clays  may  be  chemically  and  mineralogically  identi- 
cal yet  behave  widely  different  in  the  burning. 

In  the  one  all  the  minerals  may  be  in  a fine  state  of  divi- 
sion and  thoroughly  mixed  and  the  action  of  one  mineral 
upon  the  other  in  a maximum  degree  is  possible,  while  in  the 
other,  all  or  any  one  of  the  minerals  may  be  in  coarse  frag- 
ments which  would  prevent  the  close  association  essential  to 
the  development  of  a mixture  fusible  at  a lower  temperature 
than  any  of  the  minerals. 

Some  minerals  develop  more  than  one  eutectic  mixture. 
The  addition  of  one  to  the  other  in  increasing  amounts  lowers 
the  fusion  point  to  a certain  minimum.  Further  additions 
raises  the  fusion  point  for  a period,  after  which  continued 
additions  again  lowers  the  fusion  point  to  a second  eutectic 
followed  by  a second  rise. 

In  clay  wares  we  do  not  have  mixtures  of  simple  minerals 
with  which  it  is  possible  to  determine  the  eutectic  points. 
If  we  could  plot  the  eutectic  points  in  terms  of  temperature 
and  viscosity  we  would  find  sudden  drops  in  the  viscosity  curve 
as  illustrated  in  the  following  assumed  curve,  Figure  1. 

The  temperature  would  increase  perhaps  in  a straight 


12 


BURNING  CLAY  WARES. 


line.  The  viscosity  with  the  beginning  of  fusion  would  be 
very  high  and  would  decrease  with  increase  of  temperature 
but  not  in  a straight  line  nor  in  a uniform  curve.  As  eutectic 
points  “A,”  “B”  and  “C”  were  approached  there  would  be  a 
rapid  drop  in  the  curve,  but  beyond  these  points  the  down- 
ward tendency  of  the  curve  would  be  slower. 

Suppose  we  are  burning  paving  bricks  and  in  one  material 
the  soaking  heat  required  to  burn  the  bricks  to  the  bottom  of 
the  kiln  is  approaching  a eutectic  point,  the  effect  will  be  a 
rapid  softening  of  the  ware  and  distortion,  in  other  words  a 
short  vitrification  range.  In  another  material  the  final  result 
may  be  obtained  between  two  eutectic  points  where  the  vitri- 
fication range  would  be  longer,  the  rate  of  softening  would 
be  slower. 


Clay  wares,  particularly  the  common  wares,  are  not  burned 
to  complete  fusion.  Complete  vitrification  under  any  defini- 
tion applicable  to  clay  wares  is  simply  filling  the  pores  of  the 
clay  mass  with  fused  material.  Three  per  cent,  has  been 
suggested  as  the  maximum  limit  of  absorption  for  vitrified 
wares,  but  this  is  too  low.  Many  vitrified  wares  run  five  to 
six  per  cent,  absorption  and  some  as  high  as  ten  per  cent, 
and  find  acceptance  in  the  markets.  The  fusion  in  common 
wares  is  far  from  complete  and  is  stopped  short  of  the  point 
where  deformation  under  weight  becomes  serious,  otherwise 
the  ware  would  be  ruined,  but  in  vitrified  wares  we  go  so 
near  the  limit  that  the  development  or  non-development  of 
a eutectic  mixture  at  the  finishing  temeprature  may  be  the 


BURNING  CLAY  WARES. 


13 


factor  which  determines  whether  the  material  is  practical  or 
not  for  such  ware. 

By  the  way  of  an  illustration,  let  us  consider  a single  min- 
eral— lime,  for  instance — in  a clay  body.  Lime  is  very  basic 
and  an  active  flux,  but  potash  and  soda  are  even  more  active. 
Clayworkers  are  familiar  with  the  difficulty  in  burning  a 
limey  clay.  We  have  been  accustomed  to  say  that  lime  is  not 
active  at  a low  temperature  but  at  a critical  higher  tempera- 
ture it  becomes  exceedingly  active  which  results  in  the  sud- 
den failure  of  the  clay  mass  under  fire.  Any  flux  increases 
in  activity  with  increase  in  temperature  but  seemingly  at  a 


10 

o 

.'O 


Temperature 

Figure  2. 


slower  rate  than  lime.  We  know  from  the  color  of  the  ware 
that  lime  has  some  fluxing  action  at  low  kiln  temperatures. 
At  some  critical  higher  temperature  the  fluxing  action  is  so 
rapid  that  the  temperature  range  between  soft  ware  and  dis- 
torted ware  is  too  short  for  commercial  operation.  It  may  be 
simple  fluxing  action  causing  the  sudden  change,  but  it  is 
likely  that  a eutectic  mixture,  equivalent  to  a sudden  advance 
in  temperature,  is  the  cause  of  the  marked  decrease  in  vis- 
cosity. 

We  illustrate  the  effect  in  sketch,  Fig.  2,  in  which  the  upper 
curve  is  the  theoretical  viscosity  of  a simple  fluxing  action 
under  advancing  temepratures  while  the  lower  curve  repre- 


14 


BURNING  CLAY  WARES. 


sents  the  viscosity  of  the  same  flux  including  the  develop- 
ment of  a eutectic  mixture. 

Lime  with  pure  kaolin  is  known  to  develop  two  or  three 
eutectic  mixtures  while  feldspar  and  magnesia  do  not  develop 
any  eutectic.  These  tests  are,  of  course,  on  pure  materials 
and  do  not  necessarily  apply  to  such  complex  mixtures  as 
we  have  in  clay,  but  undoubtedly,  the  eutectic  tendency  of  a 
mineral  has  material  effect  on  the  burning  behavior  of  a clay 
mass. 

From  1800  to  2000  degrees  Fahrenheit  potassium,  mag- 
nesium and  calcium  minerals  may  be  acting  as  fluxes  at  the 
same  rate.  Between  2000  and  2100  degrees  the  calcium  min- 
eral may  develop  a eutectic  and  down  goes  the  ware,  while 
with  a magnesium  or  potach  mineral  no  eutectic  is  devel- 
oped and  the  rate  of  fusion  is  a continuation  of  that  at  the 
lower  temperature.  With  one  mineral  we  get  a degree  of 
fusion  within  one  hundred  degrees,  which  may  require  several 
hundred  degrees  with  either  of  the  other  minerals. 

The  bond  in  a common  clay  ware  is  the  cementing  to- 
gether of  the  grains  of  the  clay  mass.  The  grains  may  be 
coated  with  finely  divided  material  either  as  dust  in  dry 
pressed  ware,  or  sludge  in  suspension  coating  the  grains  as  the 
water  in  mud  ware  evaporates.  This  finely  divided  material  is 
the  flux  which  starts  the  fusion  and  which  hardens  in  cooling 
to  a permanent  cementing  material.  The  fused  material  will 
naturally  collect  where  the  angular  points  of  the  clay  grains 
are  in  closest  contact.  It  is  possible,  even  likely,  that  where 
the  clay  grains  are  in  close  contact  that  there  will  be  surface 
fusion  because  of  the  contact  of  the  two  surfaces,  but  it  is  of 
no  moment  whether  the  fusion  is  due  to  fine  material  coating 
the  grains  or  to  close  contact  of  the  grains. 

In  a clay  where  the  initial  bond  is  due  to  fusion  of  some 
portion  of  the  clay  mass  and  the  fused  material  serves  as  a 
cement  to  hold  the  mass  together  and  to  resist  the  action  of 
weather  agencies. 

As  the  temperature  advances  the  volume  of  fused  material 
increases  by  solution  of  the  more  refractory  minerals  and  by 
the  development  of  other  eutectic  mixtures,  and  the  pores  of 
the  clay  mass  are  filled  with  the  fused  material.  In  the 
initial  bond  we  have  an  aggregate  of  grains  cemented  together ; 
next  a fused  matrix  in  which  are  embedded  the  grains  of  more 
refractory  material;  finally  a mass  approaching  glassiness  in 
its  structure.  Some  clays  are  made  up  of  quite  similar  ma- 


BURNING  CLAY  WARES. 


15 


terials,  all  of  which  enter  into  the  fusible  mixture  and  de- 
velop a glassy  body  quickly.  Other  clays  are  made  up  of 
widely  differing  refractory  materials  and  the  most  refractory 
grains  are  slow  in  dissolving,  resulting  in  a granitoid  body. 

In  the  fusion  process  there  is  not  only  solution,  but  also 
chemical  reaction,  and  in  the  cooling  process  other  definite 
minerals  are  formed  in  the  matrix  which  may  play  an  im- 
portant part  in  the  structure  and  toughness  of  the  bonding 
mass. 

The  Chief  Minerals  in  Clay. 

The  chief  minerals  in  clay  other  than  kaolinite  and  often 
far  in  excess  of  the  kaolinite,  are : quartz,  mica,  feldspar,  iron 
as  protoxide,  sesquioxide,  sulphate,  sulphide  and  carbonate; 
lime  as  carbonate  and  sulphate;  carbon  as  coal,  bitumen,  and 
vegetable  matter;  magnesia  as  carbonate  and  silicate. 

There  is  a long  list  of  minerals  which  might  be  enum- 
erated, but  the  above  list  includes  the  minerals  which  are  the 
most  common  and  which  have  marked  effect  upon  the  burning 
behavior  of  clays. 

Kaolin. 

Kaolin,  when  pure,  fuses  at  cone  36  (3362  degrees  F.) 

Note — Cones  are  ceramic  mixtures  in  convenient  form  used 
to  denote  the  fusion  point  of  clay  bodies,  but  they  are  not  ac- 
curate in  the  determination  of  temperatures,  and  should  not 
be  used  for  this  purpose.  Cones  will  fuse  at  higher  or  lower 
temperatures  according  as  the  heat  is  applied  quickly  or  slowly 
and  the  low  temperature  cones  are  materially  influenced  by 
the  character  of  the  kiln  atmosphere.  Since  cones  are  ceramic 
mixtures  they  are  especially  valuable  in  measuring  the  tem- 
perature required  by  other  ceramic  mixtures  because  both  are 
similarly  affected  by  kiln  conditions. 

Kaolinte  is  the  refractory  base  of  clays,  although  lime  and 
magnesia  are  more  refractory  but  they  occur  as  impurities 
and,  being  strongly  alkaline,  act  as  fluxes  in  conjunction  with 
more  acid  minerals. 

Where  lime  is  excessive  in  amount,  exceeding  the  kaolin 
base,  its  action  is  that  of  a refractory,  but  in  the  ordinary 
clays  it  is  simply  a flux. 

Bauxite  fuses  at  cone  42  and  is  much  more  refractory  than 
kaolin,  but  it  should  be  considered  as  a separate  mineral, 
since  it  does  not  commonly  appear  in  clays.  Where  conditions 
have  been  favorable  for  its  development  it  is  found  under  clay 


16 


BURNING  CLAY  WARES. 


beds,  in  clay  beds  and  above  clay  beds,  and  apparently  has  been 
derived  from  the  clay,  but  its  occurrence  is  relatively  rare. 

The  property  attributed  to  kaolin  which  makes  it  valuable 
to  the  clayworker  is  plasticity,  but  as  a matter  of  fact  pure 
kaolins  are  not  highly  plastic.  All  that  can  be  said  is  that  we 
do  not  know  “why  is  plasticity.”  Impure  kaolins,  such  as  ball 


Figure  3. 


clay,  are  highly  plastic,  and  still  more  impure  clays,  such  as 
gumbo,  are  even  more  plastic.  Flint  clays,  which  in  composi- 
tion are  often  pure  kaolins,  have  no  plasticity. 

Some  very  plastic  clays  when  made  into  ware  and  dried 
develop  a very  hard  body,  almost  rock-like,  while  others  equally 
plastic  develop  relatively  tender  bodies  in  drying. 

The  illustration  No.  3 shows  the  most  plastic  clay  we  have 


BURNING  CLAY  WARES. 


17 


ever  tested.  A good  plastic  clay  flowing  from  a one-inch  die 
will  ordinarily  break  under  its  own  weight  when  unsupported 
at  about  eight  inches.  The  material  in  question,  which  was  a 
shale,  did  not  break,  but  bent  down  until  it  touched  the  ground, 
thus  giving  the  free  end  a support. 

It  was  then  run  out,  as  shown  in  illustration  No.  4,  support- 


Figure  4. 


ing  the  free  end,  and  the  bar  was  over  five  feet  long  before  it 
broke.*  The  full  length  bar  is  shown  on  the  floor  in  illustra- 
tion No.  3 with  a three-foot  rule  back  of  it.  The  dried  ware 
from  this  shale  had  good  strength,  but  nothing  unusual. 

* Since  the  above  was  written  we  have  tested  a common 
limey  clay  which,  under  the  conditions  of  Figure  4,  exceeded 
eight  feet  in  length  before  breaking. 


18 


BURNING  CLAY  WARES. 


Very  plastic  days,  such  as  ball  clays  and  gumbo,  are  very 
difficult  to  dry  without  cracking,  but  the  shale  illustrated  was 
first  class  in  its  drying  behavior. 

Our  opinion  of  plasticity  is  that  it  is  due  to  some  physical 
property  of  the  kaolin,  probably  extreme  fineness  of  grain ; 
that  the  grains  of  kaolin  possess  high  adhesive  power  and  at 
the  same  time  selective,  as  we  know  to  be  true  in  Fuller’s 
earth;  that  the  impurities  coating  the  grains  of  kaolin  serve 
as  a lubricant  when  moistened,  permitting  slippage  of  the 
grains;  that  these  impurities  when  the  clay  is  dried  serve  as 
the  cementing  material  to  bond  the  clay  mass  together  and  the 
strength  of  the  dried  ware  is  dependent  upon  the  character  of 
the  impurities;  that  the  drying  behavior  of  the  clay  mass  is 
dependent  upon  the  degree  to  which  the  pores  of  the  clay  mass 
are  filled  by  the  impurities,  or  by  colloidal  material. 

The  study  and  discussion  of  plasticity  is  greatly  interesting 
scientists,  and  it  is  to  be  hoped  that  the  result  of  their  work 
will  give  us  a theory  the  application  of  which  will  enable  us 
to  develop  or  reduce  plasticity  to  any  desired  degree,  and  to 
overcome  the  troubles  which  are  associated  with  it. 

Plasticity  is  not  a burning  problem  except  in  so  far  as  it 
develops  faults  in  the  ware  which  appear  in  the  burned  prod- 
uct and  because  of  the  burning.  It  is  the  important  property 
which  makes  clays  valuable  in  the  ceramic  industries,  and  we 
know  that  plasticity  in  the  clay  is  due  to  the  kaolin,  although 
pure  kaolin  is  less  plastic  than  many  clays.  We  are  not  con- 
cerned whether  the  plasticity  is  a property  of  the  kaolin  or 
induced  in  the  kaolin  by  salts  and  other  materials. 

Kaolin  is  a hydrous  alumina  silicate  containing  13.9  per 
cent,  of  chemically  combined  water  which  comes  off  in  the 
burning  at  temperatures  between  800  and  900  degrees  F. 

Common  clays  have  chemically  combined  water  in  propor- 
tion to  the  content  of  kaolin  or  other  hydrous  minerals,  and 
the  quantity  varies  from  to  3 per  cent,  up  to  14  or  more  per 
cent. 

Clay  takes  up  a relatively  large  amount  of  water,  commonly 
called  free  water  or  moisture  to  distinguish  it  from  the  chem- 
ical or  combined  water,  and  this  free  water  is  so  closely  held 
by  the  clay  that  seldom  can  it  be  squeezed  out  in  the  manu- 
facturing processes,  although  frequently  the  pressure  to  which 
the  clay  is  subjected  in  forming  the  ware  is  quite  heavy. 

This  free  water  is  removed  largely  in  the  drying,  but  the 
removal  of  the  last  traces  of  it  is  an  important  stage  in  the 
burning  process. 


BURNING  CLAY  WARES. 


19 


Kaolin  is  subject  to  a large  reduction  in  volume  as  the 
burning  progresses,  and  likewise  clay  in  proportion  to  the  con- 
tent of  kaolin.  Shrinkage  is  not  a property  peculiar  to  kaolin, 
but  it  is  an  important  factor  in  developing  a dense  ware  with- 
out carrying  the  burning  to  an  extreme  degree  of  fusion,  and 
it  is  fortunate  that  kaolin  possesses  this  property  in  high 
degree. 

Shrinkage  is  often  the  cause  of  considerable  loss  in  the 
burning  in  that  it  decreases  the  difficulty  of  keeping  the  ware 
in  place  and  also  in  that  provision  must  be  made  to  relieve  the 
strains  developed  by  it  to  prevent  rupture  of  the  ware,  but  at 
the  same  time  it  aids  in  the  operation  of  burning  in  that  it  is 
an  excellent  measure  of  the  progress  of  the  burn. 

Quartz. 

The  most  common  mineral  in  clay,  other  than  kaolinite,  is 
quartz,  or  silica  sand.  Quartz  is  highly  refractory,  though 
less  so  than  kaolinite,  and  it  has  an  important  place  in  re- 
fractory products. 

It  fuses  to  a glass  at  about  cone  25  (2966°  F.)  but  it  re- 
tains its  shape  to  temperatures  above  cone  30.  Its  final  fusion 
temperature  has  not  been  accurately  determined  except  to  the 
extent  that  it  is  completely  fused  at  temperatures  below  the 
fusion  temperature  of  kaolin te.  The  impression  seems  to 
prevail  among  claywTorkers  that  quartz,  or  its  equivalent  in 
sand,  is  more  refractory  than  clay  and  that  any  addition  of 
sand  will  increase  the  refractoriness  of  the  clay.  This  is  true 
of  ordinary  clays  which  have  a low  refractory  value,  but  it 
is  not  true  of  pure  clays  which  approximate  kaolinite  in  their 
composition.  On  the  contrary,  silica  when  added  to  such  pure 
clays  results  in  a mixture  which  fuses  at  a lower  temperature 
than  either  the  clay  or  the  silica  alone,  as  Seger  has  shown. 

The  addition  of  silica  to  a very  limey  clay — one  containing 
20  to  30  per  cent,  of  lime,  which  is  difficult  to  burn  because  of 
the  high  lime  content — would  serve  to  dilute  the  lime  and  at 
the  same  time  assist  in  the  development  of  a fusible  mixture. 

The  size  of  the  grain  of  the  quartz  will  have  material  in- 
fluence on  the  rate  of  fusion  and  in  a finely  divided  state  will 
more  rapidly  develop  a fusible  mixture  with  the  othei  clay 
minerals. 

Quartz,  as  sand,  is  used  to  reduce  plasticity  by  dilution,  to 
counteract  shrinkage  both  in  drying  and  in  burning,  and  aids 
in  overcoming  lamination,  particularly  if  it  is  coarse  and 
angular. 


20 


BURNING  CLAY  WARES. 


In  some  instances  fine  sand  may  lower  the  burning  tem- 
perature required,  but  as  a rule,  it  does  not  improve  the  burn- 
ing behavior,  frequently  quite  the  contrary,  while  coarse  sand 
often  shows  a decided  improvement. 

The  coarse  sand  reduces  the  lamination  of  the  clay  ware, 
increases  the  pore  spaces  which  are  important  in  the  drying, 
and  serves  as  a mechanical  bonding  material,  thus  improving 
the  quality  of  the  ware  to  be  burned,  which  results  in  better 
burned  ware. 

In  the  burning  the  large  grains  may  be  only  partially  ab- 
sorbed into  the  fused  matrix  and  the  undissolved  remnants 
serve  as  binders  and  give  the  ware  a granitoid  texture,  which 
has  the  greatest  resistance  to  shocks  and  to  temperature 
changes,  especially  in  vitrified  products.  Fine  sand  has  less 
effect  on  lamination  and  does  not  improve  the  drying  quali- 
ties, or  at  least  whatever  gain  there  may  be  in  porosity  is 
largely  offset  by  the  weaker  bond. 

In  burning  to  vitrification,  as  the  term  is  understood  in 
common  clay  wares,  the  fine  sand  is  largely  absorbed  into  the 
fused  matrix  and  the  texture  of  the  burned  product  is  glassy 
in  character  and  frequently  subject  to  heavy  loss  in  cooling 
cracks. 

Silica  under  heat  undergoes  physical  changes  which  min- 
eralogists recognize  as  distinct  mineral  forms.  To  the  clay- 
worker  the  feature  of  importance  is  expansion.  Up  to  1600 
degrees  F.  silica  decreases  in  specific  gravity  and  increases 
in  volume  about  14  per  cent.  At  higher  temperatures  it  in- 
creases in  specific  gravity  and  probably  accompanied  by  a 
corresponding  decrease  in  volume. 

Kiln  temperatures  generally  exceed  1600  degrees  F.,  and  in 
this  behavior  of  silica  may  be  a possible  explanation  why  so 
many  fine-grained  silicious  clays  do  not  have  a clear  ringing 
sound  when  burned. 

The  silica  in  the  burning  first  expands  materially  then  per- 
haps shrinks  slightly,  and  in  the  cooling  expands  slightly 
then  shrinks. 

It  is  the  cooling  expansion  which  we  would  suspect  as  the 
cause  of  the  trouble.  If  the  grains  are  round,  as  they  likely 
are  in  the  fine-grained  alluvial  clays,  the  expansion  cannot  be 
taken  up  by  the  pore  spaces  and  there  will  be  a slight  rup- 
ture of  the  bond  in  consequence.  Naturally,  such  a product 
will  not  have  the  clear  ringing  sound  of  a perfectly  bonded 
product. 


BURNING  CLAY  WARES. 


21 


Mica. 

Mica  is  found  in  nearly  all  clays  and  some  clays  are  largely 
mica,  as  for  instance,  some  of  the  micaceous  sands  of  New 
Jersey,  which  formerly  were  put  on  the  market  as  “Kaolin.” 
A sample  of  kaolin  recently  tested  had  80  per  cent,  of  mica 
and  silica,  the  mica  largely  predominating.  The  percentage 
of  kaolin  was  very  small. 

Mica  is  a silicate  of  alumina  with  some  base.  There  are 
potash,  soda,  lithia,  iron,  lime,  and  magnesia  micas  besides 
micas  containing  two  or  more  of  these  bases. 

The  clay  will  contain  the  mica  which  has  resisted  the 
weathering  influences  in  greatest  degree,  and  the  potash  mica 
— muscovite — is  the  most  common. 

Muscovite  is  said  to  fuse  at  cone  13,  which  is  above  com- 
mon kiln  temperatures.  Addition  of  kaolin  even  up  to  twenty 
per  cent,  seems  to  have  little  effect  on  the  fusibility,  in  other 
words,  does  not  readily  develop  a fusible  mixture.  This 
explains  why  mica  is  so  often  seen  in  burned  clay  wares. 
The  state  of  division  undoubtedly  has  considerable  effect  in 
producing  a fusible  mixture,  and  Stull,  Yol.  IV,  Transactions 
American  Ceramic  Society,  has  shown  that  finely  ground  mica 
exerts  a fluxing  action  below  cone  4 and  that  alone  it  vitrifies 
to  a non-absorbent  body  below  cone  4.  Reike,  a German  inves- 
tigator, on  the  other  hand,  found  that  mixtures  of  mica  and 
kaolin  in  percentages  of  twenty  per  cent,  mica  to  eighty  per 
cent,  kaolin  only  reduced  the  fusion  point  of  the  kaolin  from 
cone  35  to  cone  34.  Mixtures  containing  as  high  as  forty  per 
cent,  of  mica  only  lowered  the  fusion  point  to  cone  32.  It 
seems  inconsistent  that  a mineral  that  fuses  at  cone  13  should, 
when  mixed  in  large  amounts  with  a clay  fusing  at  cone  35, 
have  less  effect  than  the  mixture  of  two  highly  refractory 
bodies,  such  as  kaolin  and  silica,  but  the  explanation  is  that 
the  mineral  developed  bears  no  relation  in  its  fusing  point  to 
the  fusing  points  of  the  original  minerals.  This  explains  why 
the  micaceous  sands  of  New  Jersey,  mixed  with  kaolin,  pro- 
duce excellent  fire  bricks.  Stull’s  work  shows  vitrification  at 
ordinary  kiln  temperatures,  but  there  may  be  and  evidently  is 
a long  range  between  vitrification  and  fusion. 

Mica,  then,  in  a fine  state  of  division  aids  in  vitrification 
at  ordinary  kiln  temperatures  but  requires  a higher  tempera- 
ture when  present  in  plates  visible  to  the  naked  eye. 

The  point  is  that  a fine  state  of  division  permits  an  inti- 
mate mixture  and  this  is  essential  in  developing  a fusible 


22 


BURNING  CLAY  WARES. 


body.  Lumps  of  kaolin  and  quartz  in  a crucible  in  the  pro- 
portions of  the  most  fusible  mixture  will  not  fuse  at  the  tem- 
perature required  for  that  mixture  but  if  finely  ground  and 
intimately  mixed  a eutectic  mixture  is  developed. 

Mica  cannot  be  considered  as  having  any  serious  effect  in 
the  burning  behavior  but  under  some  conditions  it  will  im- 
prove the  burning  qualities. 

The  behavior  of  mica  indicates  how  variable  may  be  the 
problems  in  burning  such  complex  material  as  clay. 

Lime 

Lime  occurs  in  clays  in  several  mineral  forms — as  a car- 
bonate (limestone),  as  a carbonate  with  magnesia  (dolomite), 
as  a sulphate  (gypsum),  as  a constituent  of  silicate  minerals — 
and  it  is  a serious  menace  as  in  clay  burning  operations. 

Lime  carbonate  as  pebbles  burns  to  caustic  lime,  which, 
when  exposed  to  the  weather,  slakes,  swells  and  ruptures  the 
product.  In  large  pieces  deeply  embedded  in  the  ware,  the 
rupture  of  the  ware  through  hydration  of  the  lime  is  com- 
plete, but  small  pieces  cannot  exert  sufficient  pressure  to  cause 
rupture  except  when  near  the  surface,  and  in  such  instances 
circular  discs  are  flaked  off.  This  surface  effect  of  lime  is 
commonly  termed  ‘‘popping.” 

The  dissociation  temperature  of  lime  carbonate  is  about 
cone  014  (1526°  F.),  which  is  lower  than  commercial  kiln 
temperatures,  and  in  consequence  any  lime  carbonate  in  the 
clay  is  certain  to  be  converted  into  lime,  and  “popping” 
follows. 

Lime  sulphate  is  present  in  clays  as  gypsum,  and  it  is 
also  developed  by  the  oxidation  of  sulphide  minerals  result- 
ing in  sulphuric  acid  which  reacts  with  the  lime  carbonate 
to  form  the  sulphate.  The  dirty  white  coating  which  comes 
to  the  surface  of  clay  wares  in  the  drying  is  largely  lime 
sulphate.  Lime  sulphate  dissociates  at  higher  temperatures 
than  the  carbonate,  especially  under  oxidizing  kiln  conditions, 
and  in  consequence  in  many  kiln  operations  passes  through  the 
kiln  unchanged.  The  proof  of  this  is  the  “scum”  on  many  of 
our  burned  wares. 

Clays  which  “pop”  when  burned  in  periodic  kilns  may  not 
do  so  when  burned  in  continuous  kilns,  especially  those  which 
have  no  advanced  heating  or  water-smoking  flue,  in  which 
the  water  smoking  is  done  with  gases  ladened  with  sulphur. 
Under  such  conditions  the  lime  carbonate  is  converted  into 


BURNING  CLAY  WARES. 


23 


sulphate,  which,  as  above  mentioned,  does  not  dissociate  at 
low  temperatures,  and  at  higher  temperatures  any  lime  set 
free  enters  into  and  becomes  a part  of  a fusible  mixture  and 
is  permanently  locked  up  in  some  silicate  formation. 

Lime  in  a finely  divided  state  or  as  a constituent  of  a 
silicate  mineral  or  any  mineral  which  enters  into  a fusible 
mixture  is  a serious  element  in  clay  bodies,  because  of  the 
fusibility  of  the  mixture.  It  has  been  shown  by  investigators 
of  fusible  mixtures  that  lime  with  kaolin  develops  two  or 
three  eutectic  mixtures  defining  the  term  in  the  sense  that  it 
is  the  most  fusible  mixture  of  any  mixture  of  the  same  min- 
erals in  proportions  approximating  those  of  the  eutectic  mix- 
ture, which  is  our  interpretation  of  eutectic. 

The  effect  of  these  eutectic  mixtures,  or  if  you  please,  con- 
sider it  simply  from  the  standpoint  of  a simple  flux  becoming 
active  at  a critical  temperature,  is  a rapid  development  of 
vitrification — a decrease  in  viscosity. 

In  any  commercial  kiln  there  is  a difference  of  tempera- 
ture, and  a clay  to  be  of  commercial  value  must  produce 
marketable  ware  within  such  differences  of  temperature.  In 
up-draft  kilns  we  must  hold  the  temperature  in  the  bottom 
at  a maximum  until  the  heat  works  to  the  top,  but  we  never 
attain  the  same  temperature  in  the  top  as  that  in  the  bottom. 

Similarly  in  down-draft  kilns,  we  must  hold  the  heat  in 
the  top  until  we  can  get  the  ware  burned  to  the  bottom.  We 
consider  three  cones  the  minimum  limit  of  variation  of  tem- 
peratures in  down-draft  kilns,  and  no  other  type  of  kiln  has 
such  a small  limit. 

This  statement  requires  some  explanation.  It  is  possible 
to  attain  the  same  temperature  in  the  top  and  bottom  of  a 
down-draft  kiln,  but  at  considerable  expense  in  fuel  and  time, 
which  the  majority  of  wares  will  not  stand.  Up-and-down 
draft  kilns  develop  more  uniform  temperatures,  but  their 
application  is  limited. 

If  the  clay,  to  produce  marketable  ware,  will  only  stand  a 
range  of  two  cones,  or  one  cone,  we  must  sacrifice  the  ware 
in  one  part  of  the  kiln  to  get  good  ware  in  another  part.  This 
is  the  weakness  of  limey  clays.  The  lime  develops  mixtures 
which  fuse  so  rapidly  that  in  many  instances  it  is  impossible 
to  get  a properly  bonded  ware  throughout  the  kiln. 

Under  a subsequent  discussion  of  the  burning  behavior 
of  clays  will  be  found  curves,  in  which  are  shown  the  burning 
ranges  of  a number  of  clays,  including  limey  clays. 


24 


BURNING  CLAY  WARES. 


One  cannot  determine  how  serious  the  lime  trouble  may 
be  from  a casual  examination  of  clays.  We  have  seen  shales 
interstratified  with  limestone  in  a prohibitive  degree,  provided 
the  limestone  developed  the  usual  effect,  but  which  in  use  did 
not  “pop,”  and  the  burning  range  was  satisfactory  for  com- 
mercial operation.  Our  conclusion  was,  that  the  limestone 
was  impure,  probably  earthy,  and  that  the  granules  of  this 
material  burned  to  a lime-alumina-silicate,  in  itself  quite  in- 
fusible and  which  would  not  slake.  The  segregation  of  the 
lime  prevented  the  formation  of  a fusible  mixture  of  it  with 
the  other  clay  ingredients. 

Lime  in  small  percentages  in  clay  shows  little  effect,  at 
least  in  practical  operations.  As  the  lime  content  increases 
we  experience  the  serious  fluxing  difficulty,  but  when  it  be- 
comes excessive  it  acts  as  a refractory  and  carries  the  burn- 
ing over  the  fluxing  range ; or,  to  be  consistent,  a low  tem- 
perature fusible  mixture  is  impossible  with  such  excess  of 
lime. 

A marked  feature  of  lime  is  the  color  produced.  A clay 
containing  sufficient  iron  to  burn  red  will,  when  impregnated 
with  lime,  burn  to  a buff,  changing  at  higher  temperatures  to 
a yellow-green  and  finally  to  a decided  green.  At  very  low 
temperatures,  before  the  lime  enters  into  the  fusible  mixture 
in  any  effective  degree,  the  color  of  the  ware  is  red.  The  red 
color  is  due  to  the  iron,  and  disappears  when  the  iron  begins 
to  combine  with  the  lime  and  silica  as  a lime-iron-silicate, 
while  the  final  green  color  is  due  to  a full  development  of  the 
lime  iron  body.  The  color  and  its  mutations  are  characteristic 
of  a limey  clay. 

A limey  clay  seldom  produces  a pleasing  face  building 
color.  While  red,  buff  and  green  are  satisfactory  building 
colors,  limey  clays  are  not  satisfactory,  in  that  we  cannot 
produce  any  uniformity  of  color,  and  the  result  is  motely 
effect  in  the  wall. 

Particularly  is  this  true  in  continuous  kilns  where  the 
product  will  be  red  streaked  with  buff,  and  vice  versa. 

There  is  a large  excess  of  air  in  a continuous  kiln,  and 
the  kiln  atmosphere  is  as  nearly  oxidizing  all  the  time  as  is 
possible  in  any  kiln.  Ferric  oxide,  which  is  red  and  gives  the 
color  to  red  wares,  does  not  enter  into  chemical  combination 
in  the  ferric  state,  at  least  not  with  lime,  and  the  effect  of 
the  kiln  atmosphere  is  to  maintain  this  ferric  condition. 

It  is  a contest  between  the  oxidizing  kiln  atmosphere  and 
the  dissociating  influence  of  a fusible  mixture. 


BURNING  CLAY  WARES. 


25 


As  the  temperature  advances  the  oxidizing  conditions  are 
weakened,  and  the  effect  of  the  fusible  mixture  strengthened; 
but  in  many  instances  the  fusion  does  not  go  far  enough  to 
fully  include  the  iron,  and  in  consequence  the  streakiness  of 
the  product  is  more  marked  in  a continuous  kiln.  It  is  pos- 
sible to  overcome  this  effect  by  dampering  the  kiln  and  thus 
produce  a reducing  condition  in  the  kiln  atmosphere,  which 
reduces  the  iron  and  puts  it  in  condition  to  combine  readily 
with  the  lime  and  silica. 

The  usual  effect  of  a limey  clay  product  in  a wall  is  that 
of  a ware  that  has  been  coated  with  mortar  and  then  cleaned 
with  a scraper  or  skutch. 

There  are  exceptions,  however,  and  we  have  seen  excellent 
buff  and  green  wares  sorted  from  limey  clay  products ; nor 
must  the  pale  red  products  be  overlooked. 

In  the  latter  the  lime  mixture  absorbs  only  part  of  the 
iron,  or  possibly  some  mixture  other  than  lime  develops  the 
bond  at  a lower  temperature  than  that  of  the  lime  eutectic, 
and  the  iron  only  partially  enters  into  this  other  mixture. 
Clay  is  too  complex  to  draw  any  conclusions,  except  in  regard 
to  very  pronounced  phenomena. 

Gypsum  (lime  sulphate),  as  has  been  noted,  is  of  frequent 
occurrence  in  clays  and  also  that  it  dissociates  at  a higher 
temperature  than  lime  carbonate.  The  dissociation  begins 
below  1800°  F.,  but  becomes  very  rapid  about  this  temperature 
and  up  to  2000°  F. 

Limey  clay  products  in  the  burning  frequently  begin  to 
fail  at  about  cone  02  (2030°  F.),  and  in  consequence  the  burn- 
ing is  done  at  a lower  temperature  than  this. 

We  have  seen  in  limey  clay  products  crackled  surfaces, 
especially  where  the  ware  has  been  exposed  to  the  flame  around 
the  bag.  This  may  be  due  to  increased  shrinkage  under  ex- 
posure to  flame  temperature,  but,  the  ware  not  being  de- 
formed, the  exposure  to  such  flame  temperature  must  have 
been  only  for  short  periods,  however  many  times  it  may  have 
been  repeated. 

It  is  not  unlikely  that  the  scum  on  the  surface  of  the 
ware,  being  reduced  as  it  usually  is  around  the  bags  of  a 
kiln,  introduces  -additional  flux  into  the  surface  layers,  caus- 
ing largely  increased  shrinkage,  and  that  the  crackling  is  due 
to  this  rather  than  the  general  effect  of  the  fusible  mixture 
in  the  body  of  the  ware. 


26 


BURNING  CLAY  WARES. 


Feldspar. 

Feldspar  has  been  noted  as  one  of  the  common  minerals 
in  clay.  The  potash  feldspar — orthoclase — is  probably  most 
commonly  found  in  clays  first,  because  it  is  more  abundant  in 
granite  rocks ; and,  second,  perhaps  because  it  may  resist 
weathering  influences  in  greater  degree  than  the  other  feld- 
spars. 

Feldspar  enters  largely  in  ceramic  bodies  as  a fluxing 
mineral,  and  it  is  a desirable  mineral  in  any  clay  body.  With 
pure  clay  it  does  not  develop  any  eutectic  mixture  and  its 
effect  as  a flux  is  more  or  less  proportional  to  the  advance 
in  temperature. 

The  fusion  temperature  of  commercial  potash  feldspar  is 
listed  by  different  authorties  from  cone  4 to  cone  9 but  in 
ceramic  bodies  it  undoubtedly  develops  mixtures  fusible  at 
lower  temperatures.  Cone  4 to  cone  9,  however,  are  not  un- 
usual temperatures  in  commercial  kilns,  even  on  common 
wares,  and  felspar  must  be  ranked  as  an  important  mineral 
in  clays  in  the  development  of  a permanent  bond  and  in 
vitrification.  Fortunately,  it  is  a safe  material  for  this  pur- 
pose. 

Carbon. 

Carbon  occurs  in  clays  as  vegetable  matter,  such  at  root- 
lets, mould,  etc. ; as  bituminous  matter  high  in  volatile  gases, 
such  as  lignite,  bituminous  coal,  oil  and  oil  residues ; as  non- 
volatile carbon,  such  as  graphite.  It  also  occurs  as  a mineral 
constituent  as  in  carbonates. 

Carbon  plays  an  important  part  in  the  burning  process, 
and  it  is  often  the  cause  of  considerable  trouble,  though  in 
limited  amounts  it  may  be  beneficial. 

(1)  To  whatever  extent  it  is  present  in  the  clay  it  may 
assist  in  the  burning,  and  indeed  it  is  frequently  added  to  the 
clay  for  this  very  purpose. 

(2)  It  retards  the  oxidation  of  iron,  and  because  of  this 
factor  we  often  get  into  serious  trouble. 

(3)  It  produces  a porous  product. 

The  Hudson  River  district  and  Chicago  are  noted  examples 
of  the  use  of  carbon  for  no  other  purpose  than  to  assist  in  the 
burning.  In  one  locality  coal  dust  is  added  to  the  extent  of  a 
little  over  one  per  cent,  of  the  clay,  and  the  double  coaled 
product  which  is  for  casing,  etc.,  has  about  thirteen  per  cent, 
of  coal. 

Sawdust  is  frequently  used,  but  more  especially  to  increase 
the  porosity  of  the  product.  Coke,  anthracite  and  semi- 


BURNING  CLAY  WARES. 


27 


anthracite  are  the  usual  types  of  coal  added  to  clays,  and  they 
the  generally  safe,  while  bituminous  coal,  because  of  the 
volatile  gas,  the  rapid  combustion,  and  the  relative  high  tem- 
perature developed  are  likely  to  cause  bloating  and  black  cor- 
ing. Lignite  is  used  in  a number  of  localities  and  has  proven 
to  be  excellent  material  in  spite  of  the  fact  that  it  is  highly 
gaseous. 

These  materials,  under  proper  control,  not  only  aid  in  the 
burning,  but  they  serve  as  “grog”  to  reduce  lamination. 

The  serious  difficulty  of  carbon  in  clay  is  its  retardation 
of  the  oxidation  of  other  minerals.  If  is  a reducing  agent,  and 
until  it  has  been  burned  out  there  can  be  no  oxidation  of  the 
other  minerals. 

If  the  combustion  of  the  carbon  is  rapid  and  it  develops 
temperatures  sufficient  to  fuse  the  clay  in  contact  with  the 
carbon,  bloating  and  black  coring  result.  The  carbon  can 
only  burn  when  supplied  with  air,  and  the  fusion  of  the  clay 
mass  produces  an  impervious  body,  thus  shutting  off  the  air 
supply.  The  entrapped  intensely  heated  carbon  will  take  oxy- 
gen from  any  minerals  in  the  clay  mass  containing  oxygen  and 
the  gas  thus  developed  being  unable  to  escape  forms  bubbles 
of  blebs,  increasing  in  size  because  of  the  expansive  force 
of  the  gas  under  advancing  temperatures. 

In  burning  carbonaceous  clays  it  is  necessary  to  remove  the 
carbon  at  low  temperatures,  which  means  at  a slow  rate,  since 
the  carbon  itself  may  develop  a fusing  temperature  in  the 
clay  mass. 

There  are  many  carbonaceous  clays  which  cannot  be  burned 
in  the  ordinary  manner.  The  black  shales  in  Ohio  and  the 
black  cretaceous  clays  in  New  Jersey  are  examples  of  clays 
difficult  to  burn  because  of  their  carbon  content.  As  soon  as 
the  carbon  in  the  clay  becomes  ignited  it  burns  at  such  a rate 
as  to  produce  fusion  and  the  result  is  the  same  as  if  the  clay 
had  been  badly  overburned  in  the  ordinary  way.  In  burning 
such  clays  the  first  step  is  to  heat  up  the  mass  by  furnace 
fires  until  the  carbon  in  the  clay  is  ignited,  then  the  fires  are 
drawn  or  allowed  to  die  out,  the  furnaces  are  closed  up  and 
daubed,  thus  shutting  out  the  air  supply,  except  that  which 
may  leak  in  through  the  walls,  and  because  of  the  reduced  air 
supply  the  combustion  of  the  carbon  is  very  slow  and  high  tem- 
peratures do  not  develop  in  consequence.  After  the  carbon  is 
thus  burned  out  the  furnaces  are  again  put  into  use  and  the 
kiln  burned  off  in  the  usual  manner. 

The  bloating  is  due  entirely  to  the  gas  in  a fused  mass, 
and  the  black  core  is  partly  due  to  unconsumed  carbon  and 


28 


BURNING  CLAY  WARES. 


partly  to  reduced  iron  in  combination  with  silica,  forming  a 
black  iron  silicate. 

The  black  color  of  a clay  is  not  proof  that  the  clay  will 
be  unsafe  in  its  burning  behavior.  The  clay  may  be  refractory 
and  resist  the  fusing  tendency  of  the  carbon,  or  the  carbon 
may  be  of  such  character  or  in  such  condition  that  it-  will 
not  develop  a fusing  heat. 

All  that  can  be  said  is  that  a black  clay  must  be  re- 
garded with  suspicion  until  it  has  been  proven  to  be  safe 
burning. 

Iron  Minerals. 

Iron  is  a very  common  mineral  in  clay  and  also  in  many 
products  very  important.  It  occurs  in  a number  of  common 
mineral  forms,  such  as  oxides,  carbonate,  sulphide,  sulphate, 
and  as  a constituent  of  silicate  minerals. 

The  oxides  are  of  first  importance.  Clays  and  shales  may 
be  red  or  yellow  near  the  surface,  changing  to  blue  in  depth, 
and  these  colors  are  largely  due  to  iron,  particularly  the  red 
and  yellow.  The  red  ferric  oxide — hematite — is  the  most  per- 
manent form  and  the  most  common,  and  to  it  we  owe  the 
red  color  of  our  ceramic  products.  The  yellow  is  a hydrous 
ferric  oxide — limonite — which  burns  to  red.  The  blue  may  be 
ferrous  oxide,  but  the  presence  of  some  carbon  would  give 
the  blue  color  in  greater  degree  than  iron. 

There  are  some  very  red  clays  in  which  the  red  color  is 
said  to  be  due  to  an  algae  and  the  raw  clays  are  a deeper  red 
than  the  burned  product.  Such  clays  are  not  common,  how- 
ever, and  the  ordinary  red  clays  burn  to  a much  deeper  red 
than  the  raw  clay,  except  they  also  contain  lime.  The  depth 
of  the  red  color  in  the  burned  product  in  any  case  is  due,  first 
to  the  amount  of  ferric  oxide  present  or  produced  in  the  burn- 
ing, second  to  the  degree  of  fineness  and  dissemination  through 
the  clay  mass. 

If  we  soak  a mass  of  white  burning  clay  with  a strong  solu- 
tion of  ferrous  sulphate,  dry  and  burn  it,  we  will  get  a 
brilliant  deep  red  color;  if  we  mix  the  clay  with  powdered 
hematite  ore,  introducing  the  same  amount  of  iron  ore  as 
before,  we  will  get  a brownish  red  color,  but  far  less  deep  and 
less  brilliant  than  in  the  first  instance.  If  we  use  iron  scale, 
iron  sulphide,  or  metallic  iron  in  granular  form,  the  amount 
of  iron  still  the  same,  we  will  have  a buff  product  speckled 
with  black  spots. 

The  sulphate  of  iron  is  readily  soluble,  and  in  solution  it 
penetrates  to  all  parts  of  the  clay  mass,  and  when  the  water 
is  driven  off  each  grain  of  clay  is  coated  with  a film  of  iron 


BURNING  CLAY  WARES. 


29 


sulphate  which  dissociates  at  a low  temperature  to  ferric  oxide 
and  thus  a maximum  color  effect  is  obtained  from  a minimum 
quantity  of  iron.  The  powdered  ore  and  clay  are  merely  a 
mixture  and  may  be  likened  to  a “pepper  and  salt”  effect — 
at  least  to  red  pepper  and  salt,  and  the  coloring  effect  of  the 
iron  is  greatly  reduced.  Larger  grains  of  iron  ore  or  iron  will 
produce  red  or  black  spots,  but  the  natural  color  of  the  clay 
predominates. 

The  thin  film  of  sulphate  readily  oxidizes  to  the  ferric  state 
and  the  surface  of  a grain  of  iron  also  will  oxidize,  but  the 
oxidized  outer  face  protects  the  inner  core  from  oxidation  and 
the  result  is  a more  or  less  brown  color,  due  to  the  close 
association  of  the  black  ferrous  and  red  ferric  oxides. 

Sulphides  of  iron  burn  black  because  the  sulphur  prevents 
the  oxidation  of  the  residual  ferrous  oxide,  and  the  latter 
easily  combines  with  silica  to  form  a black  silicate. 

Iron  sulphide — pyrite,  marcasite  and  pyrrhotite — introduces 
a troublesome  problem  in  burning  clay  wares,  and  hardly 
a clay  is  entirely  free  from  this  mineral. 

It  occurs  most  frequently  as  a concretion  varying  in  size 
from  granules,  invisible  to  the  naked  eye,  to  lumps  an  inch 
or  more  in  length.  In  this  granular  form  it  has  little  effect  in 
producing  color,  even  though  in  the  burning  process  it  becomes 
ferric  oxide.  Ware  will  often  come  from  the  kiln  with  a 
flaked  or  “popped”  surface,  and  in  the  center  at  the  bottom  of 
each  disc  will  be  a red,  brown,  or  black  mineral  grain.  This 
mineral  grain  is  the  residual  of  pyrite  or  iron  carbonate 
granule. 

There  is  a reduction  in  volume  of  these  minerals  in  the 
dissociation  stages,  but  the  shrinkage  of  the  clay  mass  closes 
up  the  space.  In  the  subsequent  oxidation  of  the  iron  mineral 
the  increase  in  volume  in  consequence  of  oxidation  often  ex- 
erts enough  pressure  to  flake  off  the  surface  of  the  ware  in 
circular  discs  within  the  radius  of  influence  of  the  pressure, 
similar  to  the  behavior  of  lime  pebbles. 

The  sulphur  in  pyrite  begins  to  pass  off  at  very  low  tem- 
peratures, but  the  evolution  does  not  become  rapid  until  a 
temperature  of  about  700  deg.  F.  is  reached,  and  above  this 
temperature  the  rate  of  expulsion  increases  with  the  tempera- 
ture up  to  about  1300  deg.  F.,  but  we  cannot  completely  drive 
off  the  sulphur  at  this  temperature.  The  “blue  smoke”  from 
a kiln  stack  is  evidence  of  the  sulphur  coming  off,  and  we 
have  seen  operations  where  the  temperatures  were  advanced 
by  stages  holding  the  temperature  constant  at  each  stage  until 
the  “blue  smoke”  became  very  light,  but  which  again  became 
heavy  with  an  advance  in  the  temperature. 


30 


BURNING  CLAY  WARES. 


The  sulphur  comes  off  at  all  temperatures  and  there  are 
some  dissociation  temperatures  where  the  expulsion  is  par- 
ticularly heavy,  but  we  know  that  in  many  instances  some 
sulphur  remains  in  the  ware  up  to  the  highest  kiln  tempera- 
tures. 

In  this  dissociation  behavior  of  iron  sulphide  lies  one  of 
our  serious  burning  problems. 

After  the  ware  reaches  the  fusion  point,  the  sulphur  gas 
still  coming  off  but  unable  to  escape,  bloats  the  ware.  If  the 
grains  of  pyrite  are  near  the  surface  the  bloating  effect  will 
develop  large  blisters  on  the  surface  of  the  ware.  The  effect 
of  pyrite  in  clay  has  been  admirably  shown  by  H.  B.  Hender- 
son in  a paper  on  “Clay  Testing,”  read  before  the  National 
Brick  Manufacturers’  Association  at  the  annual  meeting,  Feb- 
ruary, 1916,  and  published  in  The  Clay-Worker  in  the  follow- 
in  March. 

We  have  mentioned  the  bloating  on  account  of  carbon  gas, 
and  sulphur  gas,  or  any  gas,  will  have  the  same  effect  under 
similar  conditions.  The  sulphur  blebs  are  likely  to  be  larger 
than  the  carbon  blebs  because  the  pyrite  granules  are  often 
larger  than  the  carbon  grains  and  a larger  volume  of  gas  is 
developed  in  the  immediate  vicinity  of  the  granule. 

Bloating,  or  vesicular  structure,  occurs  in  many  clays  when 
the  temperature  in  burning  is  carried  too  high,  and  while 
it  is  due  to  a gas  formation  it  is  not  always  sulphur  or  carbon 
gases.  In  fact,  very  few  clays  fuse  quietly  to  a dense  glassy 
body.  In  the  fusion  process  as  the  different  minerals  enter 
the  fusion  mixture  there  is  a chemical  change  taking  place 
and  frequently  a gas  given  off  in  consequence.  For  example, 
ferric  oxide — Fe202 — enters  into  combination  with  other  min- 
erals only  after  it  is  reduced  to  ferrous  oxide — FeO — and  in 
the  reduction,  or  in  the  reaction  or  fusion  which  combines  the 
ferrous  oxide  and  silica,  one  molecule  of  oxygen  is  given  off. 

Two  things  are  happening  at  the  same  time,  namely ; a 
mineral  which  does  not  readily  enter  into  a fusible  mixture 
is  being  converted  to  one  which  does  act  as  a flux,  thus  pro- 
ducing or  aiding  fusion,  and  a gas  is  being  given  off  which 
entrapped  in  the  fused  mass  causes  the  vesicular  structure. 

The  fusion  of  minerals  which  in  fusing  do  not  give  off  any 
gas  would  be  quiet  and  there  would  be  no  bleb  structure. 
Such  minerals  would  tend  to  have  a very  long,  safe  vitrifica- 
tion range  within  the  clayworker’s  definition  of  the  term,  while 
on  the  other  hand  clays  that  develop  a large  volume  of 
gas  in  fusing  and  at  the  same  time  greatly  increase  the 


BURNING  CLAY  WARES. 


31 


fusibility  of  the  fusible  mixture  by  the  addition  of  other  min- 
erals would  have  a very  short  vitrification  range.  There  are 
all  degrees  in  between  these  two  extremes. 

As  previously  stated,  the  red  color  in  our  clay  products 
is  due  to  the  ferric  iron  present  and  the  intensity  of  the  color 
is  determined  by  the  state  of  division  of  the  mineral.  If  we 
start  with  a clay  containing  ferrous  and  ferric  oxides,  or 
only  ferrous  oxide,  the  first  step  is  the  oxidation  of  the  ferrous 
oxide  to  the  ferric.  This  begins  at  a very  low  temperature 
and  the  rate  of  oxidation  increases  rapidly  up  to  a temperature 
of  about  1300  deg.  F.,  provided  there  is  no  carbon  present. 

The  carbon,  as  has  been  stated,  prevents  the  oxidation  of 
the  iron  and  must  first  be  burned  out. 

As  oxidation  proceeds  the  amount  of  ferric  oxide  increases 
as  the  ferrous  oxide  decreases,  and  in  consequence  the  depth 
of  the  red  color  increases  correspondingly.  We  will  have  the 
red  color  so  long  as  the  ferric  oxide  can  be  maintained  in  a 
free  state. 

At  high  temperatures,  even  though  there  is  an  excess  of 
air  in  the  combustion  gases,  oxidation  progresses  very  slowly, 
practically  ceases,  indeed,  some  reduction  takes  place.  The 
incomplete  combustion  gases  satisfy  themselevs  with  the  oxy- 
gen from  the  minerals  in  the  clay  mass  given  off  in  the  process 
of  fusion,  in  preference  to  the  oxygen  in  the  air  accompanying 
the  gases.  At  high  temperatures  then  the  ferric  oxide  is 
being  reduced  to  ferrous,  which  combines  with  silica  to  form  a 
black  iron  silicate.  The  color  of  the  ware  changes  from  red, 
to  red-brown,  to  brown,  to  dark  brown,  to  black.  In  the  later 
stages  of  the  fusion  undoubtedly  the  iron  in  the  silicate  min- 
erals is  having  effect  in  darkening  the  color. 

There  are  some  results  which  are  not  readily  explainable. 

In  some  clays  the  red  color  continues  even  to  complete 
vitrification ; in  others  the  red  color  changes  to  a brown  almost 
with  the  beginning  of  vitrification  and  deepens  to  a black  as 
vitrification  advances. 

If  we  color  a clay  with  a solution  of  iron — sulphate  for 
instance — a red  color  will  develop  and  continue  to  a high 
temperature,  but  if  we  use  powdered  hematite  the  brown  color 
will  appear  at  a relatively  low  temperature  and  darken  to 
black  at  higher  temperatures. 

In  other  words,  if  the  iron  is  in  a chemical  state  of  divi- 
sion the  color  is  red,  but  if  in  a mechanical  division  the  result 
becomes  black. 

We  do  not  know  whether  iron  can  go  into  combination 


52 


BURNING  CLAY  WARES. 


and  retain  its  ferric  form ; if  so,  this  would  explain  the  red 
color  under  conditions  which  ordinarily  develop,  brown  or 
black.  In  one  instance  the  ferric  oxide  may  simply  enter 
into  solution  in  the  fused  mass  without  losing  its  identity, 
and  in  the  other  instance  it  loses  its  identity  by  combination. 
It  may  be  that  in  the  chemical  state  of  division  the  iron  coat- 
ing the  grains  of  the  clay  mass  is  in  too  small  a quantity 
relative  to  the  mass  to  develop  a fusible  mixture,  or  one  that 
is  permanent,  and  in  cooling  the  iron  oxidizes  to  the  red  color, 
while  in  the  mechanical  state  of  division  such  a volume  of  the 
black  ferrous  silicate  is  formed  that  the  red  color  is  fully 
mantled.  Whatever  the  explanation,  some  clays  will  remain 
red  to  complete  vitrification  while  others  will  not. 

Magnesia. 

Magnesia  occurs  in  clays  as  a carbonate  (magnesite),  and 
associated  with  lime  (dolomite).  It  is  a common  constituent 
of  many  silicate  minerals,  and  is  nearly  always  present  in 
clays  in  small  amounts. 

It  has  been  claimed  that  it  is  an  excellent  flux  in  that 
it  acts  slowly  and  lengthens  the  vitrification  range,  reduces 
the  tendency  to  warpage,  and  produces  a tougher  product.  Its 
addition  to  paving  brick  materials  has  been  recommended. 

Our  experience  with  it  has  not  been  encouraging.  We  have 
found  that,  when  present  in  quantity  such  as  would  with  lime 
give  a very  short  vitrification  range,  its  behavior  is  very  simi- 
lar to  that  of  lime. 

It  is  better  than  lime  in  that  at  just  the  right  temperature 
an  excellent  paving  block  is  obtained,  but  the  vitrification  range 
was  too  short  to  be  practical  and  the  distortion  was  excessive. 
Our  work  is,  however,  not  conclusive.  Parmelee  and  Bleininger 
(Yol.  XVI.  Trans.  Am.  Cer.  Society)  state  that  in  porcelain 
bodies  and  slags  magnesia  in  mixtures  has  a longer  fusion 
range  than  lime  and  develops  a tougher  body  subject  to  less 
distortion.  All  tests  show  the  beneficial  effect  of  magnesia 
compared  with  lime  in  the  production  of  slag  bodies,  but  in 
vitrified  common  wares  we  do  not  carry  the  fusion  to  such  a 
degree,  and  it  remains  to  be  proven  whether  the  addition  of 
magnesia  to  a vitrifying  material  containing  neither  magnesia 
nor  lime  in  appreciable  quantities  will  lengthen  the  vitrifying 
range  and  toughen  the  resulting  product. 


BURNING  CLAY  WARES. 


33 


CHAPTER  II. 


THE  BURNING  PROCESS. 

THE  BURNING  PROCESS  may  be  divided  into  a number 
of  stages  as  follows:  (1)  Drying,  commonly  called 

watersmoking.  C2)  Oxidation  and  dehydration.  (3) 
Bonding  or  shrinkage.  (4)  Vitrification.  (5)  Annealing  and 
cooling. 


Watersmoking. 

Few  clay  wares,  particularly  the  common  wares,  are  fully 
dry  when  set  in  the  kiln.  Hygroscopic  water  is  not  driven 
off  at  atmospheric  temperature  or  even  at  the  boiling  point 
of  water ; in  fact,  wares  from  dryers  having  temperatures  as 
high  as  300  degrees  F.  seem  to  contain  some  moisture,  as 
shown  by  the  operation  of  the  kiln. 

Generally  the  wares  are  far  from  dry  through  imperfect 
dryer  operation,  and  some  wares,  such  as  dry  pressed  bricks, 
are  placed  direct  from  the  machine  into  the  kiln  wherein  the 
drying  is  accomplished. 

The  watersmoking  is  slow  or  rapid  as  the  ware  will  permit. 
Dry-pressed  wares  require  from  five  to  thirty  days  slow  dry- 
ing in  the  kiln,  from  five  to  eight  days  being  the  customary 
period.  Mud  products,  which  have  been  previously  dried,  can 
be  watersmoked  quickly  in  from  twelve  to  seventy-two  hours. 

The  watersmoking  is  accomplished  by  low  fires  in  the  kiln 
furnaces.  Wood  is  frequently  used  for  this  purpose,  to  avoid 
scumming  and  sooting. 

Scumming  is  caused  by  the  combination  of  sulphur  gases, 
moisture  and  lime  or  other  minerals  in  the  clay.  The  clay 
may  not  contain  lime  in  sufficient  quantity,  in  which  event 
there  will  be  no  scumming,  and  watersmoking  may  be  done 
with  a sulphurous  fuel,  or  it  may  be  that  scumming  is  less 
serious  than  the  cost  of  a special  fuel  to  prevent  it.  Scumming 


34 


BURNING  CLAY  WARES. 


is  largely  a dryer  trouble,  and  the  development  of  a little  more 
in  the  kiln  may  be  of  no  consequence. 

The  combination  of  cold  ware,  moisture  and  a smoky  gas 
deposits  soot  which  often  fills  the  draft  spaces  among  the 
ware  and  stops  the  draft.  This  is  a common  occurrence  in 
watersmoking  with  bituminous  coal. 

Some  factories  watersmoke  with  anthracite,  coke,  or  smoke- 
less coal  and  finish  the  burn  with  bituminous  coal,  but  many 
factories  find  it  possible  to  use  bituminous  coal  from  start 
to  finish. 

In  the  watersmoking  period  we  desire  to  heat  up  the  ware, 
evaporate  and  remove  the  moisture,  and,  to  accomplish  this, 
particularly  the  removal  of  the  moisture,  it  is  essential  that 
the  draft  be  strong. 

Unfortunately,  it  is  a period  of  weak  draft,  because  the 
kiln,  stack  and  ware  are  cold  and  the  fire  is  low. 

There  is  an  advantage  in  having  two  or  four  periodic  kilns 
connected  with  a single  stack  having  an  individual  flue  for 
each  kiln.  Under  such  an  arrangement  the  chances  of  having 
a hot  stack  to  start  the  watersmoking  are  more  favorable.  In 
some  instances  where  the  stack  is  a single  one,  it  is  provided 
with  a small  furnace  in  the  base  to  heat  up  the  stack  and  the 
gases  contained  therein.  A steam  jet  also  may  be  used  for 
the  same  purpose. 

Oxidization. 

As  soon  as  the  watersmoking  is  completed,  and,  in  fact,  in 
many  products  during  the  watersmoking,  the  temperature  is 
advanced  to  that  required  in  oxidation. 

The  several  periods  in  the  burning  process  are  not  dis- 
tinctly separate  but  always  overlap  more  or  less. 

Oxidation  begins  in  the  later  stages  of  the  watersmoking 
and  continues  into  the  shrinkage  stage,  but  the  greater  part 
of  the  oxidation  occurs  at  a low  red  heat — 800  degrees  F.  to 
1300  degrees  F.  The  oxidation  includes  the  oxidation  of  the 
carbon,  the  decarbonization  of  the  carbonates,  the  desulphuri- 
zation of  the  sulphides,  the  oxidation  of  the  ferrous  oxide  and 
the  dehydration  of  the  hydrated  minerals,  particularly  the 
kaolinite. 

Sulphur  begins  to  come  off  from  pyrite  at  low  tempera- 
tures, but  the  expulsion  does  not  become  rapid  until  a tem- 
perature of  700  degrees  F.  is  reached,  which  is  below  a visible 
red  heat.  The  greater  part  of  the  sulphur  comes  off  between 
700  degrees  and  1300  degrees. 


BURNING  CLAY  WARES. 


35 


Carbon  ignites  at  about  850  degrees  F.  and  the  oxidation 
continues  up  to  about  2000  degrees  F.  It  is,  of  course,  im- 
portant that  the  carbon  be  oxidized  at  low  temperatures ; first, 
because  the  oxidation  is  more  rapid  at  low  temperatures,  and 
second,  because  many  clays  fuse  at  lower  temperatures  than 
2000  degrees,  and  bloating  would  result  if  the  carbon  were  not 
expelled  before  fusion  begins.  It  is  also  important  that  the 
ferrous  oxides,  both  those  originally  in  the  clay  and  those  de- 
veloped from  the  sulphides  and  carbonates,  be  oxidized  before 
a fusing  temeprature  is  reached,  and  this  oxidation  cannot 
take  place  until  the  carbon  is  removed. 

The  carbonates  dissociate  between  700  degrees  F.  and  1600 
degrees  F.  (iron  carbonate,  732  degrees  F. ; magnesium  car- 
bonate, 1380  degrees  F.,  and  calcium  carbonate,  1526  degrees 
F.).  Even  the  maximum  dissociation  temperature  is  300  to 
400  degrees  below  the  finishing  temperature  of  an  average 
low  burning  temperature  ware. 

It  is  our  experience  that  dehydration  of  the  clay  base — the 
expulsion  of  the  chemically  combined  water — takes  place  be- 
tween 800  degrees  F.  and  900  degrees  F.,  but  several  authori- 
ties claim  that  dehydration  begins  at  a much  lower  tempera- 
ture. 

There  are  other  hydrous  minerals  besides  kaolin  in  a clay 
mass,  and  undoubtedly  some  of  these  dehydrate  at  much 
lower  temperatures  than  kaolin.  We  know  that  a temperature 
of  several  hundred  degrees  is  necessary  to  remove  completely 
the  hygroscopic  water,  but  the  water  may  contain  some  soluble 
salts  in  solution  or  may  easily  be  acidulated  with  sulphuric 
acid,  which  would  require  higher  temperatures  to  remove  the 
water  content  and  this  hygroscopic  water  may  be  mistaken 
for  combined  water. 

There  is  a marked  decrease  in  the  weight  of  the  mass  at 
about  850  degrees  F.,  and  this  we  attribute  to  the  dissociation 
of  the  kaolin  base.  Combined  water  and  sulphur  are  both 
coming  off  rapidly  at  about  the  same  temperature,  and  this  is 
the  “blue  smoke”  period  of  the  burning,  although  the  “blue 
smoke,”  which  is  due  to  the  sulphur,  continues  long  after  the 
combined  water  has  been  driven  off.  Nine  hundred  degrees 
is  a very  low  red  heat,  barely  visible  through  the  kiln  peep- 
hole. 

Shrinkage  Period 

The  shrinkage  or  bonding  period  covers  the  fusion  from 
the  start,  up  to  that  required  for  a thoroughly  bonded  ware, 
and  it  marks  the  burning  range  of  the  clay.  We  distinguish 
between  burning  range  and  vitrification  range  in  the  same 


36 


BURNING  CLAY  WARES. 


sense  that  we  distinguish  between  ordinary  ware  and  vitri- 
fied ware. 

The  average  common  clay  begins  to  shrink  in  a measurable 
degree  at  about  cone  010  (1742  degrees  F.),  but  seldom  is  the 
fusion  sufliciently  advanced  to  develop  a permanent  bond 
until  cone  07  (1850  degrees  F.)  is  down,  and  more  frequently 
not  until  cone  04  is  turned.  The  maximum  temperatures  are 
rather  a question  of  fuel  cost  than  the  limit  of  the  clay.  We 
have  seen  common  clay  products  which  required  a tempera- 
ture up  to  cone  10. 

Shale  products  range  from  cone  04  to  cone  5;  No.  2 fire 
clay  products  from  cone  1 to  cone  9 ; No.  1 fire  clay  from  cone 
5 to  cone  14 ; silica  and  magnesite  bricks,  cone  20.  The  tem- 
peratures are  totals  and  include  the  vitrification  range  in 
vitrified  products. 

The  vitrification  period  is  simply  an  extension  of  the  bond- 
ing period  to  a more  complete  fusion. 

Vitrification. 

The  term  vitrification  is  used  very  loosely  in  the  ceramic 
industries.  Sewer  pipe  and  paving  brick  bodies  are  vitrified, 
yet  an  examination  of  the  body  in  the  majority  of  instances 
shows  a large  percentage  of  mineral  grains  which  have  not 
entered  into  the  fusion. 

These  vitrified  bodies  will  absorb  water  up  to  10  per  cent, 
in  some  instances,  and  3 per  cent,  to  5 per  cent,  absorption  is 
of  common  occurrence. 

Porcelains  are  also  vitrified,  but  there  is  a big  gap  between 
porcelain  and  vitrified  bricks. 

The  only  reason  for  assigning  a burning  period  to  vitrifica- 
tion is  that  there  is  a definite  product  classed  as  vitrified,  such 
as  paving  bricks,  sewer  pipe  and  electrical  conduits,  and  there 
is  a limitation  in  the  clays  which  will  produce  such  products. 

There  is  no  change  in  the  burning  process — we  merely  ad- 
vance the  heat  to  the  vitrifying  point  and  hold  it  until  all  the 
ware  in  the  kiln  is  properly  burned.  It  would  be  more  proper 
to  say  heat  soaking  period,  since  the  production  of  all  hard- 
burned  ware  or  vitrified  ware  requires  a period  of  heat  soak- 
ing in  order  to  get  some  degree  of  uniformity  throughout  the 
kiln.  We  may  burn  common  bricks  and  impervious  face 
bricks  to  the  same  temperature — let  us  say  cone  1 — but  when 
this  cone  is  reached  the  common  brick  kiln  is  held  for  a very 


BURNING  CLAY  WARES. 


37 


short  period  thereafter,  while  the  face  brick  kiln  is  held  for  a 
longer  period. 

In  the  common  brick  kiln  we  may  have  a variation  of  seven 
to  eight  cones  between  the  bottom  and  top  of  the  kiln,  and 
yet  have  a satisfactory  ware  throughout  the  kiln.  In  the  face 
brick  kiln  we  must  hold  the  heat  until  the  difference  in  tem- 
perature is  reduced  to  three  or  at  most  four  cones  to  get  a 
satisfactory  product.  The  only  difference  in  vitrified  products 
is  that  the  finishing  heat  is  nearer  the  failure  point  of  the 
material,  and  in  consequence  requires  greater  care. 

In  this  connection,  we  wish  to  make  a note  in  regard  to 
fuel  consumption.  We  often  get  information  that  certain 
products  are  burned  to  cone  1,  for  example,  with  300  pounds 
of  coal  per  ton,  while  someone  else  is  doing  the  same  work 
with  200  pounds. 

Why?  Assume  that  the  coal  is  the  same,  the  kiln  the  same, 
and  the  burners  equally  efficient ; the  difference  may  be  en- 
tirely due  to  a soaking  heat,  given  by  one  and  not  by  the 
other.  There  are  differences  in  the  clay,  in  the  coal,  in  the 
kilns,  both  type  and  size,  in  the  draft,  in  the  efficiency  of  the 
burners,  and  in  the  results  to  be  obtained  from  the  kiln,  al- 
though cone  1 may  be  the  temperature  in  both  instances. 

To  make  any  comparison,  all  this  data  must  be  given  and 
taken  into  consideration. 

Flashing. 

Flashing  might  properly  be  included  as  a burning  period, 
since  in  many  instances  it  is  the  final  operation  in  the  burning. 
The  purpose  of  flashing  is  to  produce  a color  effect  different 
from  that  which  naturally  results  from  ordinary  burning. 
A flashed  color  on  red  burning  wares  is  brown  to  gun  metal 
black ; on  buff  burning  clays  a golden  yellow  to  brown. 

Flashing  is  accomplished  by  closing  the  fires  toward  the 
finish  of  the  burn,  thus  shutting  out  the  secondary  air,  re- 
sulting in  a strongly  reducing  kiln  atmosphere. 

The  common  method  is  to  begin  flashing  twelve  to  twenty- 
four  hours  before  the  finish  of  the  burning.  The  fires  are 
closed  for  a period  of  two  to  six  hours,  followed  by  a longer 
or  equal  period  of  clear  fires,  and  the  alternations  are  repeated 
until  the  end  of  the  burn.  In  some  operations  the  reducing 
conditions  are  started  earlier  in  the  burning  and  it  is  found 
that  a sufficient  depth  of  flash  can  be  obtained  in  this  way 


38 


BURNING  CLAY  WARES. 


without  carrying  the  temperatures  to  such  a high  degree,  but 
the  results  in  this  method  of  firing  are  less  brilliant  and  in 
consequence  do  not  find  the  same  favor  on  the  market. 

Flashing  in  all  its  variations  is  not  fully  understood.  The 
reducing  action  of  the  kiln  gases  will  keep  the  iron  minerals 
in  the  ferrous  oxide  state  or  convert  them  into  ferrous  oxide 
and,  as  has  been  shown,  the  iron  in  this  state  readily  enters 
into  silicate  combination,  producing  a brown  to  black  ferrous 
silicate.  This  will  explain  the  brown  to  gun  metal  black 
colors  from  red  burning  clays,  but  does  not  explain  the  golden 
yellow  colors  which  develop  on  the  face  of  buff  burning  clays. 

The  iron  content  in  many  buff  burning  clays  is  largely  seg- 
regated in  grains  and  the  ordinary  reducing  effect  is  evident 
in  the  development  of  the  black  spots  throughout  the  body  of 
the  ware,  but  the  golden  yellow  color  is  a surface  effect  and 
does  not  penetrate  the  ware.  Moreover,  only  the  faces  that 
have  been  exposed  to  the  flame  or  to  the  moving  combustion 
gases  have  the  “fire  flashed”  color.  Even  in  the  red  burning 
wares  the  color  on  the  faces  exposed  to  the  flame  is  more 
brilliant  than  those  not  so  exposed.  This  greater  brilliancy 
may  be  due  to  a greater  degree  of  fusion  but  we  believe  that 
the  contact  of  the  combustion  gases  with  the  surface  has  had 
effect  just  as  in  the  buff  burning  ware. 

It  is  said  that  flashed  colors  are  due  to  some  chemical 
change  in  the  iron  content  in  the  clay,  likely  ferrous  silicate, 
which  would  occur  under  reducing  conditions,  and  that  in  the 
cooling  there  is  a superficial  oxidation  of  the  iron  which 
would  give  the  golden  color. 

It  has  been  our  opinion  that  the  surface  flash  is  in  some 
degree  due  to  a surface  accumulation  from  the  combustion 
gases. 

We  have  seen  masses  of  iron  or  iron  slag  a half  inch  thick 
built  up  on  the  surface  of  bricks  which  have  been  exposed 
to  a flame  jet  through  a crack  in  the  bag  wall,  and  crusts  on 
the  crowns  of  high  temperature  kilns  are  familiar  to  many 
clay  workers. 

The  faces  of  heavily  flashed  bricks  have  a roughness  sensi- 
ble to  touch  which  is  not  apparent  on  the  backs.  Where  the 
flame  passes  through  a small  aperture,  as  the  checker  in 
closely  set  brick  ware,  the  flash  on  the  under  brick  is  fan- 
shaped, just  as  the  gas  would  naturally  flare  out  after  leaving 
such  an  aperture  and  the  flash  is  the  mark  of  the  moving  gas. 

Flashing  is  evidently  a flame  phenomenon,  and  is  pre* 


BURNING  CLAY  WARES. 


39 


sumably  due  to  iron.  At  least  we  may  say  that  a clay  which 
does  not  flash  readily  will  flash  if  magnetic  iron  ore  is  added 
to  it.  Clays  containing  manganese  also  flash  in  greater  de- 
gree than  the  buff  burning  clay  to  which  the  manganese  is 
added,  but  as  the  manganese  is  impure  and  contains  iron,  the 
flashing  may  be  due  to  the  iron  content  rather  than  to  the 
manganese.  Whether  the  flash  is  due  to  a chemical  change 
in  the  iron  content  in  the  clay,  to  a volatization  of  the  grains 
of  iron  in  the  clay  or  to  a surface  accumulation  from  the  gases 
is  not  important  except  the  determination  of  the  question 
might  lead  to  more  intelligent  development  of  flashed  effects. 

The  flashing  is  more  intense  the  nearer  the  ware  is  to  the 
fire,  but  this  gives  no  light  on  the  problem  because  temperature 
and  reduction  are  factors  in  producing  the  flash,  and  these 
prevail  in  higher  degree  near  the  fire. 

If  the  flashed  ware  is  cooled  very  quickly,  the  flashed  color 
does  not  appear,  and  the  flash  is  more  pronounced  ag  the  cool- 
ing is  slower,  within  reasonable  limits.  This  is  merely  a 
question  of  oxidation. 

All  kiln  burners  are  familiar  with  the  difference  in  color 
of  the  draw  tests  and  the  properly  cooled  ware.  The  browns 
cool  to  reds,  the  blue,  gray  or  green  to  buff.  This  difference 
in  color  is  more  marked  in  flashed  ware,  perhaps,  because 
of  a finer  state  of  division  of  the  mineral  which  produces  the 
flash  and  a greater  susceptibility  to  oxidation  of  the  fine  ma- 
terial. 

It  is  also  known  that  the  flash  on  a fire  clay  product  can 
be  burned  off,  in  part  at  least,  by  reburning  the  ware  under 
oxidizing  conditions. 

The  process  of  manufacture  has  considerable  effect  on  the 
flashed  color  and  only  rarely  can  the  same  color  be  produced 
on  the  same  clay  ware  made  up  by  different  processes.  Many 
dry-pressed  bricks  have  a beautiful  golden  flash,  while  the 
stiff-mud  bricks  from  the  same  clay  lack  the  golden  color  and 
often  are  dull  brown.  There  are,  however,  good  flashed  stiff- 
mud  bricks,  and  the  difference  is  only  one  of  degree  in  the 
effect  on  the  same  clay  made  by  different  processes.  The 
dry-pressed  flashed  bricks  have  retained  a strong  hold  in  the 
market  partly  because  of  the  fine  color  and  partly  because 
they  are  more  impervious  than  other  dry-pressed  bricks,  but 
the  use  of  the  plain  red,  buff  and  gray  dry-pressed  bricks  has 
been  waning. 


40 


BURNING  CLAY  WARES. 


Henderson  in  Yol.  1,  No.  3,  Jour.  American  Ceramic  So- 
ciety, has  shown  that  flashing  on  fire  clay  bodies  and  in  salt 
glazing  is  due  to  an  amber-colored  hexagonal  crystal  in  the  de- 
velopment of  which  carbon  has  played  an  important  role, 
although  the  crystal  is  not  graphite.  This  explains  several 
phenomena.  The  essential  smoky  flame  supplies  the  carbon 
which  the  surface  absorbs.  The  crystals  develop  in  cooling, 
and  hot  ware  or  ware  chilled  suddenly  will  not  show  the 
flashed  color.  If  we  remove  the  carbon  by  subsequent  oxida- 
tion the  crystals  do  not  recur  in  the  following  cooling  stage. 

Cooling. 

Cooling  is  not  a stage  in  the  burning,  but  in  many  wares 
proper  cooling  in  order  to  anneal  the  ware  is  an  important 
factor  in  the  production  of  sound  tough  ware.  When  the 
firing  ceases,  particularly  in  vitrified  wares,  the  mass  is  ra 
a state  of  semi-fusion,  and  the  rate  of  cooling  has  material 
effect  on  the  physical  structure.  Minerals  are  forming  and 
crystals  developing  which  serve  to  lace  the  mass  together 
and  give  it  greater  resistance  to  shocks.  In  some  measure 
we  may  compare  the  fused  mass  with  glass  which,  as  is  well 
known,  must  be  annealed  to  relieve  the  strains  and  give  it  the 
toughness  without  which  it  would  be  of  little  practical  value. 
We  have  held  that  the  annealing  is  accomplished  at  a low  red 
heat  and  that  we  may  safely  cool  rapidly  from  the  finishing 
temperature  down  to  a red  heat  when  the  rate  of  cooling 
should  be  slower  in  order  to  properly  anneal  the  ware.  Kiln 
burners  are  familiar  with  the  snapping  and  cracking  that  can 
be  heard  in  the  kiln  as  the  cooling  takes  place  and  these  evi- 
dences of  the  relief  of  cooling  strains  are  only  heard  in  the 
later  stages  of  the  cooling  process. 

Wares  that  are  subject  to  cooling  cracks  are  particularly 
benefited  by  thorough  annealing,  and  such  wares  should  be 
entirely  cooled  by  conduction  and  radiation  and  not  by  con- 
vection. The  kilns  should  be  closed  and  daubed  and  the 
dampers  closed  during  the  annealing  stage.  Cooling  cracks 
are  easily  recognized.  They  never  open  up  and  a casual  ob- 
servation does  not  reveal  them.  Ringing  two  bricks  together 
will  show  whether  the  ware  is  cracked  or  not,  and  if  cracked, 
an  examination  will  reveal  a fine  hair-like  crack,  sometimes 
nearly  through  the  ware.  In  fact,  the  ware  may  be  entirely 
cracked  through  and  when  picked  up  falls  into  two  pieces, 
yet  the  crack  is  not  seen  until  some  movement  has  caused  a 


BURNING  CLAY  WARES. 


41 


separation  of  the  two  pieces.  The  top  courses  in  many  paving 
brick  products  are  rejected  because  they  are  too  brittle, 
although  they  are  the  hardest  bricks  in  the  kiln,  and  except 
for  the  brittleness  should  stand  the  best  test.  We  frequently 
find  cooling  cracks  in  such  bricks,  but  they  are  difficult  to  see, 
as  the  bricks  come  from  the  kiln.  After  exposure  to  the 
weather,  when  the  cracks  have  been  developed  by  the  infiltra- 
tion of  dirty  water,  they  are  very  evident. 

Some  wares,  including  vitrified  products,  are  apparently 
not  improved  by  slow  cooling,  and  the  cooling  fans  can  be 
connected,  or  the  wickets  opened  and  the  dampers  raised  as 
soon  as  the  burning  is  finished  and  the  cooling  from  begin- 
ning to  end  carried  out  as  rapidly  as  possible.  Theoretically 
any  ware  will  be  improved  by  slow  cooling,  but  in  view  of  the 
practical  evidence  it  would  be  foolish  to  urge  slow  cooling  on 
a theoretical  consideration.  Every  manufacturer,  however, 
should  determine  whether  his  ware  is  improved  by  slow  cooling 
and  to  what  extent.  It  may  be  cheaper  to  stand  the  cooling 
losses  than  the  cost  of  the  slower  operation,  and,  of  course, 
that  which  shows  the  most  profitable  operation  should  be 
adopted. 


42 


BURNING  CLAY  WARES, 


CHAPTER  III. 

BURNING  BEHAVIOR  OF  CLAYS. 

THE  BURNING  behavior  of  clays  is  shown  by  the  color 
changes  with  advancing  temperature,  such  as : Light 

red,  red,  dark  red,  brown,  black ; light  red,  red,  dark 
red,  dark  red,  etc.,  the  red  color  continuing  even  to  complete 
vitrification ; cream  buff,  buff,  greenish  buff,  green  or  gray ; 
light  red,  buff,  greenish  buff,  green,  dark  green ; cream  white 
to  grayish  white. 

Under  reducing  kiln  atmospherers  besides  the  browns  and 
gun-metal  blacks  which  we  get  from  red  burning  clays,  there 
are  a number  of  color  effects,  such  as  saffron  and  dull  green, 
besides  the  partially  reduced  products  in  which  we  have 
brown  edges  and  red  centers. 

The  flashed  colors  tend  to  the  russets  in  buff  burning 
clays,  usually  speckled  with  black  iron  spots,  while  the  red 
burning  clays  have  the  browns  and  gun-metal  blacks.  The 
color  is  primarily  due  to  the  character  of  the  clay,  but  the 
variation  in  color  marks  the  progress  in  the  burning.  Burners 
are  instructed  to  burn  this  kiln  hard  for  gun  metals,  that 
kiln  for  reds,  or  red  centers,  another  for  a light  flash,  a heavy 
flash,  bluish  buffs,  light  or  dark  grays,  etc.,  etc. 

The  difficulty  of  burning  is  dependent  upon  the  character 
of  the  clay.  A clay  with  a very  short  burning  range  requires 
great  care  in  getting  the  temperature  high  enough  to  pro- 
duce satisfactory  ware,  yet  without  exceeding  the  safe  burn- 
ing limit  of  the  clay.  Face  building  wares  are  troublesome 
because  of  the  wide  variety  of  color  effects  which  must  be 
produced  and  which  require  different  burning  treatment. 

Vitrified  wares  require  temperatures  approaching  the  max- 
imum limit  and  the  only  favorable  factor  is  that  each  kiln 
receives  the  same  treatment.  Clays  which  are  used  for  com- 


BURNING  CLAY  WARES. 


43 


mon  wares,  such  as  common  brick,  drain  tile,  fire-proofing, 
etc.,  are  often  impractical  for  face  brick  products  or  vitrified 
wares,  but  on  the  other  hand,  clays  which  are  suitable  for 
the  latter  products  are  easily  burned  when  made  into  the 
former  products. 

The  burning  behavior  of  clays  is  nicely  illustrated  by  their 
change  in  size  under  increasing  temperatures,  as  the  following 
illustrations  show. 


The  shrinkage  is  indicated  in  per  cent,  on  the  vertical  line 
on  the  left.  The  zero  line  marks  the  original  size  of  the  test 
piece.  The  advancing  temperatures  are  shown  from  left  to 
right  on  the  base  line  and  are  indicated  by  cones. 

Illustration  Fig.  5 shows  an  ideal  shrinkage  curve  indica- 
tive of  the  burning  behavior.  At  cone  07  the  shrinkage  has 
advanced  nearly  1 per  cent,  and  at  a rate  of  about  1 per 
cent,  per  cone  advance  in  temperature.  The  rate  of  shrink- 


44 


BURNING  CLAY  WARES. 


age  decreases  gradually  until  at  cone  5 the  shrinkage  is  com- 
plete and  there  is  no  change  in  the  size  from  cone  5 to  cone  7. 
We  do  not  know  how  much  higher  the  temperature  could  be 
carried  without  damage  to  the  ware.  The  ware  is  steel  hard 
at  cone  04  and  the  burning  range  for  all  hard  ware  is  from 
cone  04  to  cone  7 — in  all  ten  cones  and  perhaps  more.  A 
burner  would  have  to  be  very  careless  or  incompetent  not 
to  get  good  results  from  such  a clay. 


Compare  this  with  Fig.  6.  The  latter  begins  to  bond  and 
shrink  at  a lower  temperature  than  No.  5,  as  is  evident  from 
the  fact  that  the  shrinkage  is  over  2 y2  per  cent,  at  cone  07. 
At  cone  02  the  curve  is  upward,  which  means  that  shrinkage 
has  ceased  and  swelling  begun.  At  cone  3 the  sample  has  lost 
5 per  cent,  of  the  total  shrinkage  through  bloating.  The  maxi- 
mum safe  burning  temperature  is  cone  02.  The  burning  range 
would  b*>  <rom  cone  02  down  to  some  lower  temperature  at 
which  salable  ware  could  be  produced. 


BURNING  CLAY  WARES. 


45 


Two  cones  are  marked  as  the  burning  range,  but  this  ap- 
plies to  the  color.  Common  ware  in  which  the  color  is  not 
important  would  be  available  probably  as  low  as  cone  07, 
which  would  give  a range  of  five  cones.  A feature  of  this  clay 
is  the  red  color  which  is  retained  through  the  marked  fusion 
up  to  cone  3.  The  behavior  of  the  clay  is  common  to  many 
clays,  the  chief  difference  being  in  the  rate  of  the  shrinkage, 
the  suddenness  of  the  change  when  the  maximum  shrinkage 
Is  reached,  and  the  rate  of  the  swelling.  The  first  two  of 
these  differences  generally  determine  the  burning  range. 


Figure  7 is  an  unusual  clay.  At  cone  07  it  is  2 per  cent, 
larger  than  the  original  size.  This  increase  in  size  is  not  due 
to  bloating,  but  to  some  mineral  in  the  clay,  which  has  caused 
the  increase  in  size  by  expansion.  The  temperature  reaches 
cone  05  before  the  sample  has  returned  to  its  original  size, 
and  the  net  total  shrinkage  is  only  1 per  cent.  At  cone  1 
failure  begins  and  continues  at  a uniform  rate  up  to  cone  5, 
when  it  would  appear  that  the  fusion  has  become  quiet. 

The  behavior  of  this  clay  raises  a number  of  questions 
in  answer  to  which  one  can  only  speculate.  What  mineral 
could  cause  such  marked  expansion  at  such  low  temperatures? 


46 


BURNING  CLAY  WARES. 


Silicious  clays  would  have  such  expansion,  but  as  a rule, 
silicious  clays  require  higher- temperatures  to  bond.  It  will 
be  noted  that  this  clay  was  hard  at  cone  07  and  if  the  expan- 
sion is  due  to  silica  there  must  be  associated  with  it  some 
mineral  which  fuses  at  a low  temperature  or  which  develops 
a low  temperature  fusible  mixture. 

The  bloating,  as  has  been  stated,  is  due  to  gas  expansion 
in  a fused  mass  but  the  query  is,  what  put  an  end  to  the 


effect  of  the  gas  at  cone  5 to  cone  7.  Did  the  viscosity  of  the 
mass  decrease  to  such  an  extent  that  the  gas  bubbles  could 
break  through  whmh  would  cause  a collapse  counteracted  by 
the  formation  of  additional  bubbles?  One  can  conceive  of  a 
constant  size  through  a short  temperature  range  under  such 
conditions.  Was  a new  mineral  forming  at  cone  5 which 
absorbed  the  gas  and  thus  preserved  the  balance? 

The  burning  range  of  this  clay  for  a good  color  would  be 


BURNING  CLAY  WARES. 


47 


cone  04  to  cone  1,  where  failure  begins,  but  for  common  wares 
the  range  would  be  from  cone  1 down  below  cone  07,  we  do 
not  know  how  far. 

Figure  8 illustrates  the  behavior  of  two  clays.  The  upper 
curve  is  that  of  a sandy  red  clay  which  shrinks  1 per  cent,  up 
to  cone  04,  but  at  higher  temperatures  up  to  the  limit  of  the 
test  tnere  is  no  further  change.  The  product  is  all  soft,  all 


Figure  9. 


light  red  and  all  porous.  This  clay  will  not  produce  satisfac- 
tory ware  except  at  temperatures  above  cone  7,  and  at  such 
higher  temperatures  the  fuel  cost  in  burning  becomes  consid- 
erable. 

The  lower  curve  shows  an  exceptional  clay.  At  cone  07 
the  shrinkage  is  over  6 y2  per  cent.  We  wonder  at  how  low 
a temperature  the  shrinkage  began  and  what  was  the  rate  of 
shrinkage.  It  will  be  noted  that  at  cone  07  the  shrinkage  was 
stationary,  and  remained  so  up  to  cone  04.  If  it  is  stationary 


48 


BURNING  CLAY  WARES. 


several  cones  below  cone  07,  what  a remarkable  material  it 
would  be  for  fire-proofing — in  that  the  product  would  be  hard, 
in  that  the  temperature  of  burning  low  and  the  fuel  consump- 
tion light,  in  that  the  ware  would  be  absolutely  the  same  size, 
and  in  that  the  burning  range  is  exceptionally  long. 

Above  cone  07  the  shrinkage  is  only  l1/^  per  cent,  up  to 
cone  7,  and  we  do  not  need  to  consider  temperatures  below 
cone  07  to  show  the  excellence  of  the  material. 


Figure  10. 


Vitrification  began  at  cone  1 and  there  was  not  the  slightest 
evidence  of  failure  up  to  cone  7. 

Even  though  cone  7 should  prove  to  be  the  limit,  yet  a 
clayworker  would  be  content  with  a vitrification  range  of  six 
cones. 

Figure  9 presents  nothing  new  except  the  sudden  drop  in 
the  curve  at  cone  04,  which  one  might  assume  marks  the  de- 
velopment of  a fusible  eutectic  mixture. 


BURNING  CLAY  WARES. 


49 


Ordinarily  a sudden  drop  at  such  an  advanced  stage  in  the 
shrinkage  would  indicate  that  the  failure  point  was  being  ap- 
proached, but  the  eight  subsequent  cones  did  not  develop  any 
failure. 

Figure  10  is  a characteristic  behavior  of  a limey  clay.  Some 
begin  shrinking  at  lower  temperatures  and  do  not  shrink  as 
much  as  the  clay  illustrated,  although  the  high  shrinkage  is 
not  unusual.  The  feature  of  the  curve  is  the  very  rapid 


shrinkage  and  the  very  sudden  change  at  the  failure  point. 
The  change  is  usually  so  sudden  that  it  is  impossible  to  get  a 
measurement  after  the  fusion  begins.  The  burning  range  for 
this  clay  will  be  between  cone  1 and  cone  5,  but  it  is  doubtful 
if  a satisfactory  range  for  an  all-hard  product  is  possible.  It 
would  be  necessary  to  stop  the  advance  of  the  temperature  be- 
fore cone  5 was  reached,  and  it  would  require  a temperature 
above  cone  1 to  insure  hardness.  The  safe  burning  range 
reduces  to  a minimum. 


50 


BURNING  CLAY  WARES. 


Figure  11  is  an  entirely  different  material.  It  is  a clay  low 
in  iron  and  contains  no  lime.  The  shrinkage  begins  below  cone 
07,  and  the  samples  are  steel  hard  at  this  temperature.  At 
cone  1 the  vitrification  is  practically  complete,  but  there  is 
no  failure  even  up  to  cone  7,  and  likely  above.  There  is  a 
vitrification  range  of  six  cones  and  probably  more,  and  a 
burning  range  of  thirteen  cones  and  more. 

The  curves  show  the  wide  variation  in  the  burning  be- 
havior of  clays,  and  they  also  show  how  futile  are  conclusions 
drawn  from  any  single  sample  test,  which  would  give  a result 
for  only  one  temperature. 

Many  clays  will  produce  good  wares  if  burned  to  just  the 
right  temperature,  but  they  may  or  may  not  produce  good 
wares  at  other  temperatures,  upon  the  possibility  of  which  de- 
pends the  commercial  value  of  the  clay. 

We  must  have  not  only  a good  quality  of  ware,  but  also  a 
satisfactory  range  of  practically  equally  good  quality;  we 
must  have  a good  color  and  also  a satisfactory  color  range; 
uniformity  in  size  is  important,  and  this  requires  a slow  rate 
of  shrinkage  within  the  temperatures  producing  good  quality 
wares. 

Salt  Glazing. 

Salt  glazing  was  not  given  as  a burning  stage  but  it  may 
very  properly  be  included.  The  ware  is  salt  glazed  during 
and  following  the  vitrification  period. 

It  is  essential  that  some  degree  of  vitrification  be  obtained 
before  salting  begins,  otherwise  the  glaze  will  be  absorbed  into 
the  body  of  the  ware  and  a satisfactory  surface  glaze  would 
only  be  possible  after  the  pores  of  the  ware  were  filled,  which 
would  require  an  excessive  amount  of  salt  and  a long  firing 
period  during  the  salting.  A salt  glaze  is  merely  a soda- 
silica-alumina  glass  coating  the  surface  of  thje  ware.  Salt  is 
used  for  the  purpose  because  of  its  abundance  and  cheapness ; 
because  it  decrepitates;  because  it  fuses  and  volatilizes  at  a 
low  kiln  temperature  (1400  degrees  F.)  ; because  it  does  not 
change  chemically  in  fusion  or  volatilization. 

Water  is  said  to  be  necessary  for  the  decomposition  of  the 
salt  and  the  reaction  is  given  as  follows:  2NaCl+H20=2HCl+ 
Na20. 

Knett,  however,  claims  that  the  water  reaction  is  merely 
secondary  and  not  essential.  His  theory  is  that  the  vaporized 
salt  reacts  with  the  iron  or  iron  oxides  and  forms  the  salt 
glaze  and  volatile  ferric  chloride  as  shown  in  the  following 
equation : (Al,Fe,)2O3,SiO2+6NaCl:=(Al,Na3)2O3,+SiO2+Fe2Cl0 


BURNING  CLAY  WARES. 


51 


The  latter  is  decomposed  by  steam  as  follows : 
Fe2Cl6+3H20=Fe208+6HCl. 

His  theory  requires  that  the  silica  be  associated  with  oxides, 
— preferably  iron  oxide. 

The  limits  of  the  ratio  of  alumina  to  silica  within  which  a 
good  glaze  is  possible  as  determined  by  Barringer  are  1 alum- 
ina to  4.6  silica  for  the  minimum  and  1 alumina  to  12.5  silica 
for  the  maximum. 

If  we  allow  15  per  cent,  for  water  and  alkalies  the  composi- 
tion of  the  minimum  ratio  will  be  as  follows : 


Silica  62.00% 

Alumina  23.00% 

Alkalies,  etc . 15.00  % 

The  composition  of  the  maximum  ratio,  allowing  12  per 
cent,  for  alkalies  and  water  will  be : 

Silica  77.50% 

Alumina  12.50% 

Alkalies,  etc 12.00% 


It  is  generally  believed  that  fine  grained  silica  is  best  for 
salt  glazing,  but  Barringer’s  tests  show  practically  no  differ- 
ence in  this  respect,  except  that  the  finer  grained  silica  gives 
a lighter  colored  glaze  which  after  all  may  be  due  to  the  fact 
that  it  is  a better  glaze  and  mantles  the  dark  body  color. 

The  best  glaze  temperature  has  not  been  determined. 

It  is  said  that  the  best  glazing  temperatures  are  between 
cone  3 and  cone  8 or  higher,  but  it  must  be  admitted  that  good 
sewer  pipes  are  made  in  kilns  which  do  not  at  any  time  at- 
tain a temperature  of  cone  3 and  fall  considerably  below  this 
during  the  salting,  and  we  have  factory  records  showing 
average  kiln  temperature  of  cone  02.  We  can  unquestionably 
get  good  glazes  if  the  temperature  is  above  cone  3,  provided 
the  other  conditions  are  right,  but  many  of  us  are  interested 
in  knowing  how  much  below  cone  3 a good  glaze  is  possible. 

The  quantity  of  salt  varies  widely,  due  probably  to  bad 
firing  conditions,  to  a ware  which  does  not  take  salt  readily, 
and  to  lack  of  vitrification  at  the  time  of  salting. 

In  some  instances  a single  salting  is  sufficient  to  develop 
the  glaze,  but  more  common  practice  is  from  three  to  six 
rounds  of  salt  firing  with  an  intermediate  fire  to  raise  the 
temperature.  Vaporizing  salt  absorbs  heat  just  as  the  vapor- 
izing of  water  and  this  heat  is  taken  from  the  fire  which  other- 


52 


BURNING  CLAY  WARES. 


wise  would  go  into  the  kiln  to  maintain  the  temperature.  If 
the  salting  is  started  at  a minimum  temperature,  or  the  tem- 
perature is  reduced  to  a minimum  in  consequence  of  the  salt- 
ing, then  before  a second  application  of  salt  the  kiln  tem- 
perature must  be  raised,  but  if  the  initial  salting  temperature 
is  in  excess  of  that  required,  then  there  may  be  several  appli- 
cations of  salt  without  any  intermediate  firing. 

The  salting  period  varies  from  three  to  twenty-four  hours 
and  the  quantity  of  salt  varies  accordingly. 

The  fires  should  be  cleaned  before  beginning  the  salting 
and  the  kiln  temperature  raised  to  a maximum,  and  as  pre- 
viously stated  the  ware  must  be  at  least  partially  vitrified. 
When  the  salting  begins  the  dampers  are  closed  to  get  a back 
draft  at  the  furnaces,  or  at  least  a balanced  draft.  The  pur- 
pose is  to  let  the  salt  fumes  drift  into  and  spread  to  all  parts 
of  the  kiln  and  to  give  time  for  the  action  of  the  fumes  on  the 
minerals  in  the  ware.  The  salt  fumes  will  clear  off  in  ten 
to  fifteen  minutes,  when  a second  application  of  salt  is  made, 
or,  as  in  the  more  common  practice  the  dampers  are  raised  and 
the  furnaces  fired  with  coal  which  is  allowed  to  burn  down  to 
a clear  fire  before  the  second  round  of  salt  is  made,  and  this 
process  is  repeated  as  many  times  as  the  practice  on  the  yard 
in  question  has  shown  it  to  be  necessary. 

In  the  majority  of  yards  from  three  to  six  hours  covers 
the  salting  period. 

A number  of  authors  state  that  in  some  localities  it  is 
customary  to  add  oil,  resin,  or  other  quick  burning  material 
with  the  salt,  presumably  for  the  purpose  of  maintaining  the 
salting  temperature,  and  perhaps  to  give  a darker  color  to 
the  ware,  but  we  have  never  seen  this  done  in  this  country  and 
we  do  not  believe  it  is  common  practice.  We  have  seen  coal 
dust  thrown  with  the  salt  into  natural  gas  fired  furnaces  for 
the  purpose  of  darkening  the  pipe,  but  later  this  practice  was 
superceded  by  the  use  of  a smoky  gas  flame  during  the  salting. 

There  are  many  interesting  problems  in  salt  glazing  and 
much  work  must  be  done  before  the  process  becomes  an  exact 
one,  and  meanwhile  each  manufacturer  must  work  out  the 
problems  for  his  particular  material  regardless  of  the  practice 
in  other  factories. 

There  are  numerous  difficulties  which  must  be  overcome, 
some  of  which  are  common  and  pretty  well  understood, — such 
as  smoked  pipe,  blown  or  slabbed  pipe,  blisters,  pimples,  craz- 
ing and  cracking,  scumming,  light  and  dark  glazes. 


BURNING  CLAY  WARES,. 


53 


Smoked  ware  has  a lustreless  gun  metal  black  color  due 
to  excessive  reducing  conditions  during  which  colloidal  car- 
bon is  absorbed  by  the  glaze.  Smoked  ware  often  shows  an 
iridescence,  which  may  be  due  to  sulphur,  and  possibly  the 
sulphur  gases  under  reducing  conditions  has  had  something 
to  do  with  the  dullness  of  the  glaze. 

Blowing  or  slabbing  occurs  in  the  watersmoking  period  of 
the  burning  and  is  said  to  be  caused  by  the  development  of 
steam,  the  pressure  of  which  forces  off  large  slabs  from  the 
surface  of  the  ware.  We  think  the  trouble  goes  back  to  the 
press  work  and  that  it  is  primarily  caused  by  lamination 
planes  in  the  ware. 

The  drying  is  done  in  low  temperature  rooms  and  is  far 
from  complete,  and  subsequent  rapid  drying  in  the  kiln  may 
result  in  the  removal  of  the  moisture  from  the  surface  faster 
than  it  can  come  to  the  surface  which  would  cause  the  outer 
layers  to  become  hard  dry,  shrink,  and  crack  loose,  though 
Garve  has  shown  that  cracking  occurs  while  the  ware  is  still 
wet  dark  and  that  there  is  little  danger  after  the  ware  becomes 
white  dry  or  hard  dry  on  the  surface  even  though  the  core  is 
still  quite  soft.  With  the  crack  once  started,  the  tendency 
of  the  layer  to  straighten  out  would  cause  a large  patch  of 
the  surface  layer  to  peel  off. 

Movement  of  the  pore  water  from  the  center  to  the  sur- 
face is  not  necessarily  a factor.  Hygroscopic  water  does  not 
travel  to  the  surface  by  capilarity  but  instead  is  driven  out 
by  penetration  of  the  heat  into  the  ware  and  the  vaporization 
of  the  moisture  from  the  surface  of  the  clay  grains.  Some 
shrinkage  accompanies  the  expulsion  of  the  hygroscopic  water 
and  if  the  operation  is  carried  on  too  rapidly  there  will  result 
the  same  hardening  and  cracking  of  the  surface  layers,  and 
the  result  is  the  same  as  a simple  drying  crack. 

Blisters  may  be  due  to  entrapped  air  in  the  lamination 
planes  which  is  under  pressure  in  passing  through  the  die,  and 
the  subsequent  expansion  puffs  up  the  surface.  Such  blisters 
develop  in  the  green  pipe. 

Blisters  in  burned  pipe  are  often  due  to  the  gases  given 
off  by  pyrite,  or  other  minerals,  which  cannot  escape  in  con- 
sequence of  the  vitrification,  or  subsequently  because  of  the 
glaze  coating,  and  their  expansion  under  advancing  heat,  or 
their  increased  pressure  in  consequence  of  increased  volume, 
causes  the  blisters. 

Incomplete  oxidization  would  account  for  this  trouble  in 


54 


BURNING  CLAY  WARES. 


some  instances,  but  as  has  been  noted,  it  is  a difficult  matter 
to  completely  oxidize  the  sulphur,  and  vitrification  may  de- 
velop gases  from  other  minerals  which  would  cause  the  blis- 
ters. Perfectly  bonded  ware,  thoroughly  oxidized,  and  not 
too  intensely  vitrified  and  burned  under  oxidizing  conditions 
as  much  as  possible,  will  reduce  the  blistering  to  a minimum. 

Pimples,  or  rough  pipes,  are  due  to  the  reduction  of  the 
iron  minerals  in  or  near  the  surface  of  the  ware  and  their 
combination  with  silica  by  fusion. 

The  fused  mass  naturally  assumes  a globular  form  pro- 
jecting from  the  surface  of  the  ware. 

The  globule  is  formed  under  reducing  conditions  and  may 
be  softened  and  absorbed  by  the  body  or  the  glaze  under  oxi- 
dizing conditions.  A ware  that  pimples  badly  should  be  sub- 
jected to  alternating  reducing  and  oxidizing  conditions  until 
the  globules  have  been  fully  developed  and  absorbed. 

Yogt  states  that  he  has  in  numerous  instances  overcome 
the  trouble  by  filling  the  furnaces  with  fine  coal  after  the  salt- 
ing is  finished.  In  this  way  he  would  get  a slow  fire  and 
Strongly  reducing  conditions  which  could  be  maintained,  per- 
haps, without  damage  to  the  ware,  and  which  would  result 
in  the  full  development  of  the  globules  and  maintaining  them 
in  a soft  condition  until  they  were  absorbed. 

Crazing  and  cracking  are  caused  either  by  too  rapid  heat- 
ing up  during  the  watersmoking,  which  would  tend  to  develop 
fine  drying  cracks  on  the  surface  and  crack  the  sockets,  and 
these  cracks  would  not  heal  in  the  burning  nor  be  covered  by 
the  glaze.  Such  cracking  is  easily  distinguished  from  ordinary 
crazing,  in  that  the  glaze  penetrates  the  cracks,  and  it  is  evi- 
dent that  the  cracking  occurred  before  the  glaze  was  developed. 

True  crazing  of  the  glaze  is  due  to  the  cooling,  or  it  may  be 
that  the  body  is  too  dense  and  cracks  under  temperature 
changes. 

It  requires  no  stretch  of  imagination  to  assume  that  the 
glaze  may  not  fit  the  body.  Barringer  suggests  that  the  glaze 
may  vary  with  the  composition  of  the  body,  and  it  very  likelv 
does.  We  would  thus  have  high  silica  content,  high  alumina, 
high  lime,  etc.,  in  the  glaze,  which  would  explain  why  some 
glazes  craze  badly  and  others  do  not.  Where  the  glaze  is  liable 
to  crazing  it  is  very  important  that  the  ware  should  be  care- 
fully cooled  in  order  to  get  a maximum  annealing. 

Scummed  ware  does  not  take  a good  glaze  simply  because 


BURNING  CLAY  WARES. 


the  salt  fumes  do  not  combine  with  the  sulphate  of  lime  and 
cannot  get  through  the  scum  to  get  at  the  underlying  silica 
and  alumina  necessary  to  develop  the  glaze. 

The  color  of  a salt  glaze  is  affected  by  the  character  of  the 
kiln  atmosphere  as  well  as  by  the  composition  of  the  body.  The 
natural  color  of  a salt  glaze  is  a golden  yellow.  Under  reduc- 
ing conditions  it  absorbs  carbon  and  the  color  darkens  to  a 
brown  and  even  to  a dull  black.  If  the  body  is  dark,  as  in 
shale  pipe,  the  color  of  the  pipe  surface  is  correspondingly  dark, 
due  to  the  dark  background,  although  it  is  likely  that  the  glaze 
will  contain  more  iron  and  have  a deeper  color  than  the  glaze 
on  fire  clay  ware. 

We  have  salt  glazed  the  same  clay  in  continuous  and  down- 
draft  kilns,  getting  from  the  former  a straw  yellow,  and  from 
the  latter  a dark  brown,  due  to  the  difference  in  the  kiln  atmos- 
pheres. 

Bloating  and  black-coring,  the  cause  of  which  has  been 
explained,  are  difficulties  which  may  occur  in  any  clay  ware 
where  the  oxidation  has  not  been  satisfactorily  carried  out  in 
the  burning  process,  and  while  this  trouble  often  occurs  in 
salt-glazed  ware  it  is  not  because  of  the  glaze  except  in  so  far 
as  the  glaze  may  seal  the  pores  of  the  body  and  prevent  the 
escape  of  the  entrapped  gases.  The  thin  glaze,  however,  could 
not  develop  sufficient  strength  to  resist  the  pressure  required 
to  bloat  the  ware  and  the  effect  of  the  glaze  will  be  negligible. 

It  is  evident  that  for  salt-glazed  ware  a long  safe  vitrifica- 
tion range  is  necessary,  and  in  order  to  get  this  range,  salt- 
glazed  products  are  often  made  from  mixtures  of  several  clays. 
Fire  clays  are  most  commonly  used  alone  because  many  of 
them  are  silicious,  yet  retain  good  plasticity,  and  are  safe 
drying,  besides  having  a long  vitrification  range.  They  also 
stand  a temperature  which  readily  develops  a good  salt  glaze. 

Some  shales  are  excellent  in  this  regard,  but  the  majority 
of  low  temperature  materials  require  a mixture  in  order  to 
get  a suitable  salt  glaze  body. 

Plastic  clays  give  the  needed  mobility ; sandy  clays  the 
requisite  silica  for  salt  glazing;  refractory  clays  lengthen  the 
vitrification  range  and  hold  the  ware  in  shape  during  the  salt- 
glazing period  when  the  failure  point  is  near,  and  yet  the  ware 
must  be  held  under  the  requisite  temperature  to  develop  the 
glaze. 


56 


BURNING  CLAY  WARES. 


CHAPTER  IY. 


FUEL  AND  COMBUSTION. 

IF  CLAYWORKERS  had  a better  understanding  of  the  prin- 
ciples of  combustion,  there  would  be  economy  in  fuel  and 
likely  better  results.  Combustion  is  simply  another  name 
for  oxidation  generally  applied  to  rapid  oxidation  wherein  the 
evolution  of  heat  is  sufficiently  rapid  to  overcome  radiation, 
convection  and  conduction  losses  and  thus  develop  temperatures 
required  in  burning  processes. 

The  usual  combustibles  are  carbon,  hydrogen  and  sulphur, 
but  it  may  be  a surprise  to  some  to  learn  that  there  are  many 
elements  which  may  be  classed  as  combustibles,  although  ordi- 
narily the  rate  of  combustion  is  so  slow  that  we  call  the  process 
oxidation  rather  than  combustion. 

The  operation  of  the  Bessemer  converter  illustrates  the 
combustion  of  elements  not  classed  as  combustibles,  but  which, 
under  the  Besemer  treatment,  develop  an  intense  temperature. 
Carbon  burns  out  first,  as  would  be  expected,  then  follows  the 
silicon  and  finally  the  iron  itself,  and  it  is  to  these  elements 
that  the  converter  owes  its  high  temperatures. 

In  clay  ware  burning  we  are  only  concerned  with  the  com 
bustlon  of  carbon,  hydrogen  and  sulphur. 

The  combustion  agent  is  oxygen,  and  it  may  come  either 
from  the  air  or  from  the  minerals  in  the  clay. 

When  we  have  reducing  kiln  atmospheres  which  we  use 
to  develop  certain  color  effects,  there  are  unburned  or  only 
partially  burned  gases  which  are  seeking  oxygen  and  which 
take  oxygen  from  the  minerals  in  the  clay — reducing  them, 
as  we  say.  If  we  expose  iron  to  an  oxidizing  atmosphere,  it 
is  converted  into  ferrous  oxide,  and  finally  ferric  oxide;  but 
if  the  latter  is  exposed  to  a reducing  atmosphere,  it  gives  up 
one  molecule  of  its  oxygen  and  becomes  ferrous  oxide,  and 
the  ferrous  oxide  may  be  reduced  to  iron;  but  ordinarily  in 
clay  wares  the  ferrous  oxide  readily  fluxes  with  silica  to  form 


BURNING  CLAY  WARES. 


57 


silicates,  of  which  there  are  several  definite  forms,  and  these 
are  more  difficult  to  dissociate  than  the  simple  oxides. 

The  fuels  are  coal,  as  anthracite,  coke,  bituminous  coal, 
and  lignite ; peat ; wood ; gas,  natural  and  artificial ; oil  and 
oil  residues. 

Anthracite  and  coke  are  chiefly  carbon,  not  considering  the 
ash.  Bituminous  coal  and  lignite  are  made  up  of  carbon  and 
hydro-carbon.  For  example,  if  we  enclose  anthracite  or  coke 
in  a crucible  without  air  the  temperature  may  be  carried  to 
an  indefinite  degree  without  materially  lessening  the  weight  of 
the  anthracite.  If  bituminous  coal  or  lignite  is  similarly  sub- 
jected to  heat,  a gas  is  driven  off  which  consists  of  hydrogen 
combined  with  carbon,  while  the  residual  coke  which  we  called 
fixed  carbon  remains.  Anthracite  is  practically  all  fixed  carbon, 
but  as  we  descend  in  the  scale  through  semi-anthracites,  semi- 
bituminous,  bituminous,  lignite,  the  fixed  carbon  decreases  and 
the  volatile  hydro-carbons  increase  in  volume. 

Natural  gas  is  a mixture  of  free  hydrogen  and  hydro- 
carbons, though  there  may  be  at  times  a small  percentage  of 
the  hydro-carbons  in  combination  with  oxygen. 

Artificial  gas,  which  is  a distillate  from  coal,  is  largely 
hydro-carbon  gases,  while  producer  gas,  which  is  derived  from 
the  imperfect  combustion  of  coal,  is  a mixture  of  hydro-carbons 
and  carbon  oxides,  the  latter  largely  predominating. 

Oils  are  hydro-carbons  associated  with  oxygen,  nitrogen  and 
sulphur  in  combination. 

Peat  and  wTood  have  large  percentages  of  cellulose,  which 
is  a hydrogen-oxygen-carbon  combination. 

While  the  composition  of  fuels  is  very  complex,  yet  in  the 
processes  of  combustion  it  reduces  to  a few  comparatively  sim- 
ple forms,  and  it  is  possible  to  approximately  determine  the 
value  of  the  fuel  by  a formula  applied  to  the  ultimate  compo- 
sition of  the  fuel. 

Carbon  first  burns  to  carbon  monoxide  and  the  latter  to 
carbon  dioxide.  If  the  combustion  is  complete,  carbon  dioxide 
is  the  resultant  gas,  and  in  such  instances  we  need  only  con- 
sider it.  Hydro-carbons  break  up  and  combine  with  oxygen 
to  form  water  and  carbon  dioxide,  and  similarly  the  hydrogen- 
oxygen-carbons  are  finally  resolved  into  water  and  carbon 
dioxide.  Sulphur  burns  to  sulphurous  anhydride,  which  further 
oxidizes  and  combines  with  water  to  form  sulphuric  acid. 

It  matters  not  what  fuel  we  are  burning,  the  resulting  gases 


68 


BURNING  CLAY  WARES. 


are  carbon  dioxide,  water  and  sulphuric  anhydride  or  acid. 
With  these,  of  course,  will  be  the  nitrogen  from  the  air,  and 
free  oxygen  to  whatever  extent  it  is  present  in  excess  of  the 
combustion  requirement.  If  the  combustion  is  incomplete,  as 
under  reducing  conditions,  and  especially  in  producer  work, 
the  gases  will  be  hydrogen,  hydro-carbons,  carbon  monoxide, 
carbon  dioxide,  water  vapor,  nitrogen  and  sulphurous  acid. 

Dulong’s  formula  for  the  calculation  of  the  value  of  a fuel 
from  an  analysis  is  as  follows:  8080XC+ 34460X(H — y8 0)-f 
2250xS=calories,  in  which  C equals  carbon,  H equals  hydro- 
gen, O equals  oxygen,  S equals  sulphur. 

A calorie  is  simply  a measure  of  heat. 

Unfortunately,  there  are  several  terms  in  use  for  the  meas- 
ure of  heat  units  and  considerable  confusion  arises  in  conse- 
quence. Values  of  fuels  are  usually  given  either  in  calories 
(C.)  or  British  thermal  units  (B.  t.  u.),  commonly  called  “heat 
units.”  Technical  books,  engineering  hand  books,  etc.,  tell  us 
that  the  calorie  is  3.968  times  the  B.  t.  u.  and  herein  we  get 
confused. 

As  a matter  of  fact,  the  only  difference  is  in  the  thermom- 
eter reading,  and  the  calorie  is  1.8  larger  than  the  B.  t.  u., 
because  it  is  measured  in  centigrade  degrees  instead  of  Fah- 
renheit. A heat  unit  is  the  heat  required  to  raise  a unit  of 
water  one  degree  from  the  maximum  density  temperature ; a 
calorie  is  the  heat  required  to  raise  one  kilogram  of  water  one 
degree  centigrade ; a B.  t.  u.  is  the  heat  required  to  raise  one 
pound  of  water  one  degree  Fahrenheit.  On  the  basis  of  these 
definitions,  which  are  correct,  a calorie  is  3.968  times  a B.  t.  u., 
but  we  do  not  use  the  terms  in  that  way,  and  the  student  who 
attempts  such  use  of  them  gets  into  trouble. 

A pound  of  fuel  will  raise  the  -temperature  of  a pound  of 
water  to  a certain  number  of  degrees  Fahrenheit,  and  this 
number  of  degrees  is  called  the  B.  t.  u.  of  the  fuel. 

We  could  use  a kilogram,  or  a ton  of  fuel,  to  raise  the  tem- 
perature of  a kilogram,  or  a ton  of  water,  and  we  would  get 
the  same  number  of  B.  t.  u. ; but  when  we  say  that  a given 
weight  of  fuel  raises  the  temperature  of  an  equal  weight  of 
water  to  a certain  number  of  centigrade  degrees,  then  the 
result  is  in  calories. 

The  calorie  and  B.  t.  u.  values  as  we  use  them  are  simply 
measures  of  temperature,  and  one  can  be  converted  into  the 
other  by  the  same  factors  that  we  use  in  converting  centigrade 
into  Fahrenheit. 


BURNING  CLAY  WARES. 


59 


We  say  that  coal  has  a heat  value  of  8,000  calories,  or 
14,400  B.  t.  u.,  and  the  latter  is  1.8  times  the  former,  or  we 
may  divide  the  B.  t.  u.  by  1.8  and  have  the  result  in  calories. 

Dulong’s  formula  above  given  is  in  calories.  It  gives  re- 
sults very  closely  approximating  the  actual  tests  of  the  fuel 
for  coals  high  in  carbon,  but  varies  considerably  from  the 
actual  results  where  oxygen  is  a constituent  of  the  fuel. 

If  the  oxygen  is  combined  with  the  carbon,  the  heating 
value  will  be  higher  than  the  formula  determination.  The 
formula  assumes  that  the  oxygen  is  combined  with  the  hydro- 
gen from  which  it  is  deducted,  whereas  it  should  be  deducted 
from  the  carbon  in  the  molecular  ratio  of  C to  02,  to  whatever 
extent  it  is  in  combination  with  carbon  in  the  fuel.  The  heat- 
ing value  of  the  coal  group  is  very  closely  approximated  by 
the  formula,  and  since  this  is  the  clay  worker’s  chief  fuel,  modi- 
fications of  the  formula  are  not  necessary  for  the  clayworker’s 
use.  The  Dulong  formula  is  applicable  to  any  coals,  and  clay- 
workers  may  determine  the  relative  value  of  their  fuels  by 
this  formula. 

A high  percentage  of  sulphur  is  always  objectionable,  in 
that  it  usually  involves  a corresponding  high  percentage  of 
iron  which  readily  combines  with  the  silica  of  the  ash  to  form 
clinker.  It  matters  not  what  the  heating  value  of  the  fuel  may 
be,  the  value  depreciates  if  the  ash  fluxes  and  shuts  off  the  air 
supply  and  requires  additional  labor  to  keep  the  grates  in  con- 
dition for  good  combustion. 

Anthracite  is  a very  dense  fuel,  and  in  consequence  does 
not  burn  readily.  It  is  necessary  to  maintain  a deep  bed  of 
the  fuel  on  the  grates  in  order  to  offset  radiation  losses  and 
the  cooling  effect  of  the  entering  air.  The  air  enters  through 
the  grates,  and  in  its  initial  contact  with  the  glowing  coals  the 
oxygen  takes  up  carbon  and  carbon  dioxide  is  formed  (C02), 
but  as  this  gas  rises  through  the  bed  of  fuel,  it  is  reduced  to 
carbon  oxide  ( CO ) . Whether  that  is  the  exact  reaction  or  not 
is  of  no  importance.  We  know  that  we  have  escaping  from 
the  top  of  the  fuel  bed  carbon  dioxide,  carbon  monoxide,  hydro- 
gen and  nitrogen,  besides  some  water  vapor  and  a little  sul- 
phurous anhydride. 

The  carbon  dioxide  is  a final  product,  but  the  carbon  mon- 
oxide is  still  a combustible  gas.  This  is  the  mixture  we  get  in 
producer  gas,  and  the  purpose  of  the  process  is  to  produce  as 
little  carbon  dioxide  and  as  much  carbon  monoxide  and  hy- 
drogen as  possible.  Anthracite  coal  is  especially  valuable  in 


60 


BURNING  CLAY  WARES. 


the  production  of  producer  gas,  in  that  the  gas  carries  very 
little  tar  products  and  sulphur  and  is  largely  carbon  monoxide. 

In  the  grate  operation  the  combustion  is  completed  by  the 
admission  of  secondary  air  over  the  top  of  the  bed  of  glowing 
coals.  Many  furnaces  have  been  devised  in  order  that  tlfe 
secondary  air  introduced  shall  be  heated  before  it  enters  the 
combustion  chamber,  because  cold  air  chills  the  gases  below 
the  ignition  point.  We  are  accustomed  to  regard  combustible 
gases  as  explosive  and  err  in  regard  to  the  ignition  point.  They 
require  higher  temperature  for  ignition  than  wood  or  coal. 

At  low  temperatures  carbon  monoxide  cracks  to  form  car- 
bon dioxide  and  carbon,  and  the  carbon  thus  formed  does  not 
readily  combine  with  oxygen,  but  instead  passes  off  as  smoke 
or  builds  up  carbon  deposits  in  the  furnace  regardless  of  tem- 
perature and  excess  of  air. 

L.  Babu,  “Traite  de  Metallurgie  Generate,”  is  authority  for 
the  statement  that  carbon  monoxide  dissociates  into  carbon 
dioxide  and  carbon  almost  completely  at  450  degrees  C.  (842 
degrees  F.),  but  the  dissociation  is  almost  zero  at  1,000  degrees 
C.  (1,832  degrees  F.).  In  order  then  to  get  the  full  benefit  of 
the  combustible  gas  rising  from  the  bed  of  coals,  it  is  essen- 
tial that  the  air  introduced  for  secondary  combustion  be  pre- 
heated in  order  to  maintain  a high  furnace  temperature. 

The  fuel  bed  for  anthracite  must  be  deep  for  reasons 
already  given,  and  also  because  the  coal  does  not  swell  in 
burning,  but,  on  the  contrary,  as  combustion  proceeds,  there 
is  a shrinkage  in  volume  and  a gradual  settling  down  of  the 
bed,  which,  if  shallow,  would  soon  develop  holes  through  which 
an  excess  volume  of  air  would  pass,  at  the  same  time  robbing 
the  deeper  portions  of  the  fuel  bed.  An  extreme  condition 
would  be  a number  of  large  holes  (black  spots)  through  which 
a large  volume  of  air  is  passing,  while  the  limited  volume  of 
air  passing  through  the  hot  coals  would  develop  a small  amount 
of  carbon  monoxide,  and  the  result  would  be  a large  excess  of 
air,  which  would  have  to  be  heated  at  the  expense  of  the  lim- 
ited volume  of  carbon  monoxide,  and  in  consequence  the  furnace 
temperature  would  be  low. 

Herein  is  the  cause  of  a great  loss  to  clayworkers,  espe- 
cially in  the  operation  of  down-draft  kilns. 

They  are  trying  to  heat  a large  volume  of  air  in  competi- 
tion with  the  sun,  without  any  return  in  hardening  the  wares 
in  the  kilns. 


BURNING  CLAY  WARES. 


61 


Anthracite  is  a short  flame  fuel,  and  because  of  its  density 
it  is  slow  burning.  On  account  of  the  latter  factor,  in  order 
to  consume  the  oxygen  from  the  air  and  get  temperature  re- 
sults, it  is  necessary  to  expose  a large  surface  of  the  fuel  to 
combustion  conditions,  and  this  means  a deep  bed  of  fuel. 

The  products  of  combustion  from  a shallow  bed  will  be 
carbon  monoxide  and  nitrogen.  The  first  result  is  wasteful  in 
dioxide  and  nitrogen ; from  a very  deep  bed,  carbon  dioxide, 
carbon  monoxideand  nitrogen.  The  first  result  is  wasteful  in 
fuel ; the  second  is  difficult  to  obtain  and  maintain ; the  third 
requires  secondary  air  to  complete  the  combustion,  and  when 
carried  to  an  extreme  is  a gas  producer  condition. 

In  order  to  avoid  the  first  condition  we  tend  toward  the 
third,  and  many  of  our  furnaces  are  crude  gas  producers,  par- 
ticularly the  inclined  grate  bar  and  grateless  furnaces. 

The  average  temperature  of  the  surface  of  a bed  of  hard 
coal  is  higher  than  that  of  bituminous  coal,  because  the  mois- 
ture and  volatile  gases  to  be  dissociated  are  much  less,  and 
in  consequence  the  heat  which  becomes  latent  in  this  com- 
bustion of  the  bituminous  coal  is  available  as  sensible  heat 
in  the  combustion  of  the  hard  coal.  The  gases  arising  from 
the  hard  coal  bed  are  quickly  ignited  in  the  furnace  if  sec- 
ondary air  is  available  and  we  have  difficulty  in  getting  the 
heat  over  into  the  kiln,  while  with  the  bituminous  coal  the 
unburned  gases  are  carried  into  the  kiln  and  burned  in  contact 
with  the  ware ; or,  in  other  words,  we  say  the  bituminous  coal 
lias  a long  flame. 

Forced  draft  is  especially  valuable  in  the  combustion  of 
anthracite,  in  that  it  increases  the  rate  of  combustion  which 
is  essential  because  of  the  density  of  the  coal. 

The  use  of  a steam  jet  also  has  possibilities,  first,  in  that 
it  gives  the  forced  draft,  and,  second,  because  of  the  deep  fuel 
bed  a maximum  percentage  of  water  gas  is  developed. 

Hydrogen  has  a combustion  value  of  about  62,028  B.  t.  u., 
and  since  a pound  of  water  contains  one-ninth  pound  of  hydro- 
gen the  dissociation  of  the  water  will  consume  about  6,892 
B.  t.  u.  If  the  furnace  temperature  is  3,000  degrees,  there 
will  be  in  addition  to  the  heat  by  dissociation,  the  specific  heat 
of  the  hydrogen  and  oxygen,  which  roughly  will  be  1,800 
B.  t.  u.,  making  a total  of  8,692  B.  t.  u.,  which  every  pound  of 
dissociated  water  will  carry  over  into  the  kiln  from  the  fur- 
nace. Air  heated  to  the  same  temperature  will  carry  about 
800  B.  t.  u. 


62 


BURNING  CLAY  WARES. 


Bituminous  coal  has  a tendency  to  cake  and  swell  in  the 
combustion  process.  When  freshly  fired,  the  volatile  constit- 
uents are  driven  off,  and  these  must  be  brought  in  contact  with 
hot  secondary  air  to  insure  combustion.  The  remaining  coke  is 
increased  in  volume  compared  with  the  coal,  and  in  conse- 
quence closes  up  the  air  spaces  and  thus  shuts  off  the  supply 
of  initial  air.  Because  of  this  the  fuel  bed  must  be  shallow 
and  uniformly  spread  over  the  grate  to  get  results.  Not  all 
bituminous  coals  are  caking,  but  the  tendency  of  all  is  in  this 
direction,  and  the  firing  conditions  are  somewhat  alike,  although 
not  identically  so.  Two  types  of  furnaces  are  in  general  use 
for  such  coals — the  flat  grate  and  the  coking  table. 

The  former  is  too  well  known  to  need  description,  and  the 
latter,  too,  although  many  clayworkers  are  using  a coking 
table  principle  and  do  not  know  it. 

The  regular  coking  table  is  a flat  surface  upon  which  the 
fresh  fuel  is  fed  and  where  the  volatile  gases  are  driven  off. 
These  gases  mingle  with  the  air  that  is  admitted  over  the 
coking  table,  and  the  mixture  passes  over  the  bed  of  glowing 
coals  and  is  burned.  Before  adding  fresh  fuel  the  coke  re- 
maining on  the  table  is  pushed  off  into  the  pit,  where  a deep 
bed  of  fuel  is  maintained,  and  properly  so  because  the  coke  is 
practically  fixed  carbon,  and  the  proper  burning  conditions  are 
those  required  for  coke  or  hard  coal. 

The  most  common  type  of  furnace  is  the  inclined  grate  bar 
furnace,  which  is  essentially  a coking  table  and  pit.  When  the 
freshly  fed  fuel  is  placed  on  the  upper  ends  of  the  bars,  it 
goes  through  the  coking  process  just  as  it  would  on  a coking 
table,  and  with  successive  firings  it  is  worked  down  into  the 
deep  bed  of  coals  in  the  ash  pit. 

The  dry  bituminous  coals  and  lignite  do  not  cake  and  there 
is  a marked  shrinkage  in  volume  from  the  time  of  firing.  As 
they  run  high  in  volatile  matter,  it  is  essential  that  the  fuel 
bed  be  relatively  deep  in  order  to  maintain  hot  coals  for  the 
combustion  of  the  gases. 

Natural  gas  and  oil  firing  to  the  uninitiated  usually  give 
a lot  of  trouble  at  first.  The  combustion  largely  takes  place 
near  the  burner,  and  the  result  is  an  intense  heat  in  the  fur- 
naces which  does  not  go  over  into  the  kiln,  especially  the  down- 
draft  kiln. 

The  furnaces  for  gas  and  oil  should  be  narrow  and  small, 
since  a large  combustion  space  is  not  needed,  and  it  is  neces- 


BURNING  CLAY  WARES. 


G3 


sary  to  leave  large  openings  around  the  burners  for  the  ad- 
mission of  air  to  take  the  heat  from  the  furnaces  into  the  kiln. 

Each  type  of  fuel  and  each  fuel  requires  special  treatment 
in  order  to  get  the  best  results,  and  no  treatise  on  the  sub- 
ject could  possibly  direct  how  to  get  results  out  of  any  fuel 
selected  at  random. 

The  combustion  gases,  including  carbon,  and  their  reactions 
are  as  follows: 

Carbon,  y2  C2+%  02=C0. 

Carbon.  y2  C2+02=C02. 

Carbon  monoxide,  CO-f  %02==C02. 

Hydrogen,  H 2+y2  02=H20. 

Methane,  CH4+202=C02+2H20. 

Ethylene,  C2H4+ 302=2C02+2H20. 

Sulphur,  S + 02=S02. 

There  is  a series  of  hydro-carbons,  but  unless  the  combus- 
tion is  very  faulty  they  do  not  appear  in  the  gaseous  products 
ol  combustion,  and  the  hydro-oxygen-carbons  are  readily  de- 
composed and  result  is  carbon  monoxide,  carbon  dioxide  and 
water. 

The  heat  obtainable  from  a fuel  depends,  of  course,  upon 
its  composition.  The  combination  of  carbon  with  oxygen,  of 
hydrogen  with  oxygen,  of  sulphur  with  oxygen  develop  a cer- 
tain amount  of  heat  units,  and  if  we  know  the  composition  of 
the  fuel  we  can  determine  the  amount  of  heat. 

The  heat  generated  by  a pound  of  carbon  burned  to  CO  is 
4,374  B.  t.  u. ; of  carbon  to  C02,  14,544  B.  t.  u. ; of  CO  to  C02, 
4,359  B.  t.  u. ; of  hydrogen  to  H20,  62,028  B.  t.  u. ; of  methane 
to  C02  and  H20,  26,315  B.  t.  u. ; ethylene  to  CaO  and  H20, 
21,327  B.  t.  u. ; sulphur  to  S02,  3,906  B.  t.  u. ; sulphur  to  S03, 
5,040  B.  t.  u. 

In  making  calculations  of  heat  values,  one  must  consider 
the  condition  of  the  resulting  gas.  For  instance,  in  the  above 
values,  it  is  assumed  that  the  HaO  returns  to  water,  but  as 
vapor  it  retains  the  latent  heat  and  the  values  of  hydrogen 
ana  the  hydro-carbon  as  above  given  must  be  corrected  in  esti- 
mating the  heat  given  up  for  kiln  purposes,  because  there  is 
no  condensation  in  the  kiln  or  kiln  stacks.  For  example,  a 
pound  of  hydrogen  produces  nine  pounds  of  water,  the  latent 
heat  of  which  at  212  degrees  F.  is  970  B.  t.  u.  per  pound,  and 
the  heat  value  as  given  above  must  be  reduced  8,730  B.  t.  u. 
Similarly  in  the  methane  there  are  2%  pounds  of  water  vapor, 
and  in  the  ethylene  1 3/7  pounds. 


64 


BURNING  CLAY  WARES. 


The  sulphur  values  given  consider  sulphur  as  a gas  in 
which  form  we  expect  to  find  it  in  kiln  gases. 

The  heat  losses  in  kiln  burning  are  distributed  as  follows : 

Evaporation  of  Water. 

(1)  Evaporation  of  moisture  in  the  fuel. 

(2)  Unburned  carbon  in  the  ash. 

(3)  Heat  of  the  ash. 

(4)  Incomplete  combustion. 

(5)  Excess  volume  of  air. 

(6)  Heat  of  escaping  gases. 

(7)  Radiation  and  leakage  losses. 

The  impression  seems  to  prevail  that  moisture  in  the 
coal  or  in  the  ash  pit  increases  the  heat  development,  but 
the  contrary  is  the  case.  Every  pound  of  water  in  the  fuel 
or  added  to  it  requires  970  B.  t.  u.  (at  212  degrees)  to  evap- 
orate it.  If  the  ware  requires  1,000  pounds  of  coal  per  unit 
thousand  of  the  ware  and  the  coal  contains  10  per  cent, 
moisture,  about  eight  pounds  of  the  coal  must  be  used  to 
evaporate  the  water.  Nor  is  this  all.  The  thermal  capacity  of 
water  vapor  is  about  twice  that  of  the  other  ordinary  combus- 
tion gases  for  any  given  temperature,  and  this  heat  is  taken 
out  of  the  kiln.  Nine  hundred  pounds  of  dry  coal  will  give 
more  available  heat  than  1,000  pounds  of  the  same  coal  con- 
taining 10  per  cent,  moisture. 

If  the  vapor  rises  through  a bed  of  glowing  coals  it  is  dis- 
sociated and  develops  water  gas  (H,+CO),  and  this  dissocia- 
tion, relative  to  the  hydrogen,  will  require  the  same  amount 
of  heat  as  that  given  up  by  the  recombination  (combustion) 
of  the  hydrogen,  but  we  lose  the  excess  thermal  capacity  of 
the  water  vapor. 

The  only  excuse  for  the  use  of  water  gas  is  that  it  con- 
sumes a lot  of  furnace  heat  in  the  dissociation  of  the  water 
vapor  and  conveys  it  into  the  kiln,  where  it  is  given  up.  In 
other  words,  water  gas  is  valuable  as  a carrier  of  heat  from 
the  furnace  to  the  kiln,  and  in  many  instances  we  can  afford 
to  pay  something  for  this  service ; but  there  is  a limit,  and 
moreover,  to  be  of  any  value  whatever,  the  water  vapor  must 
be  dissociated.  If  it  is  not  dissociated  every  pound  of  water 
takes  up  970  B.t.u.  and  the  vapor  removes  from  the  kiln  spe- 
cific heat  approximately  double  that  of  other  gases. 

Wood  contains  moisture  up  to  80  per  cent.,  peat  60  per 
cent,  or  more,  lignite  50  per  cent.,  bituminous  coal  20  per  cent, 
and  less,  anthracite  5 per  cent.  These  moisture  contents  must 


BURNING  CLAY  WARES. 


65 


be  reckoned  with  in  securing  economical  combustion  opera- 
tions. 

Unburned  Carbon  in  the  Ash. 

Bleininger’s  tests  of  four  industrial  kilns,  Trans.  A.  C.  S., 
Vol.  X,  gives  us  the  following  data : 

Per  Cent.  Per  Cent.  Per  Cent.  Per  Cent. 
Ash  Carbon  Coal  Lost  Heat  Lost 

In  Coal  In  Ash  In  Ash  In  Ash 


Sewer  pipe  kiln 11.74  29.17  3.42  4.58 

Paving  brick  kiln 14.09  21.53  3.03  3.09 

Terra  cotta  kiln 7.59  20.92  1.60  1.90 

Terra  cotta  kiln 6.55  20.53  1.34  1.60 

Brick  kiln ...  3.51 


The  average  fuel  loss  in  a clay-burning  kiln  is  easily  4 per 
cent,  of  the  coal  fired,  which,  on  the  basis  of  1,000  pounds  of 
coal  per  unit  thousand  of  ware,  would  be  40  pounds  of  coal. 
If  a factory  is  using  25  tons  of  coal  per  day,  it  is  paying  for 
one  ton  to  be  thrown  away  and  hiring  men  to  haul  it  to  the 
kiln,  shovel  it  into  the  furnaces,  rake  it  out  of  the  ash  pits, 
wheel  it  to  the  dump  pile,  and  finally  haul  it  away,  besides 
committing  a sin  for  which  posterity  must  suffer. 

The  miserable  grates  that  are  used  in  brick  kilns,  with 
warped,  sagged  and  irregularly  spaced  bars,  cause  a loss 
which  might  easily  be  turned  into  a profit.  The  purchasing 
department  keeps  a record  of  the  cost  of  grate  bars,  but  there 
is  no  record  of  coal  in  the  ash  dump. 

Heat  of  the  Ash. 

Coals  average  10  per  cent,  ash,  and  this  falls  through  the 
grate  bars  at  a glowing  heat — let  us  assume  1800  degrees  F. 
For  every  unit  thousand  of  ware  requiring  1,000  pounds  of 
coal  there  will  be  100  pounds  of  ash.  If  the  specific  heat  of 
the  ash  is  .16,  there  will  be  16  B.t.u.  per  degree  per  unit  thou- 
sand of  ware,  or  a total  of  28,800  B.t.u.,  which  is  equivalent  to 
2 y2  pounds  of  average  coal. 

A part  of  this  heat  is  recovered  in  preheating  the  air, 
which  must  flow  in  over  the  ash  before  rising  through  the 
grate  bars.  The  only  point,  then,  is  to  have  sufficient  depth 
in  the  ash  pit,  that  the  ashes  may  accumulate  and  give  up 
their  heat  to  the  incoming  air.  A shallow  ash  pit,  which  ne- 
cessitates removal  of  the  ashes  after  each  fire,  or  at  frequent 
intervals,  to  save  the  grate  bars,  is  an  unnecessary  waste  of 
fuel. 

Incomplete  Combustion  and  Excess  Air. 

The  great  kiln  losses  come  from  incomplete  combustion, 
excess  air  and  radiation. 


66 


BURNING  CLAY  WARES. 


If  we  put  a gas  coal  or  any  hydrocarbon  combustible  in  a 
closed  vessel  and  heat  it,  we  can  drive  off:  the  gas  and  lead 
it  away  to  a gas  holder  without  recovering  any  heat  from  it 
except  its  sensible  heat.  If  we  burn  this  fuel  in  our  furnaces 
without  the  proper  amount  of  air,  some  portion  of  the  gas 
passes  through  the  kiln  unburned  and  we  lose  its  value.  Every 
clayworker  is  familiar  with  the  “tassels”  of  flame  at  the 
chimney  tops,  and  usually  is  pleased  in  consequence  of  the 
“long  flame”  through  the  kiln.  He  would  be  less  pleased  if 
he  appreciated  the  fact  that  the  “tassel”  is  not  the  tail  of  the 
flame  from  the  furnaces,  but  simply  the  unburned  gases  burst- 
ing into  flame  as  they  come  in  contact  with  the  air,  either 
at  the  top  of  the  stack  or  in  consequence  of  leakage  of  air 
through  the  stack  walls.  Likewise,  a clear,  transparent  gas 
from  the  chimney  tops  is  no  evidence  of  economy  in  fuel  nor 
of  temperature.  The  kiln  may  be  taking  in  a large  excess 
of  air,  heating  it  to  a high  degree  and  expelling  it  through  the 
stack. 

The  following  table  shows  the  temperatures  which  are  possi- 
ble from  a bituminous  (Illinois)  coal  under  different  air 
supplies  and  radiation  losses  up  to  50  per  cent.  The  table 
was  calculated  after  the  method  given  by  E.  Damour,  in  “In- 
dustrial Furnaces  and  Methods  of  Control”: 

Centigrade  Temperatures  from  Bituminous  Coal. 


AIR  Assumed  Radiation  Loss. 


None 

10% 

20% 

30% 

40% 

50% 

60% 

1338 

1219 

1098 

975 

850 

720 

70% 

1617 

1480 

1339 

1193 

1042 

886 

80% 

1755 

1610 

1459 

1303 

1143 

974 

90% 

1861 

1708 

1553 

1390 

1219 

1041 

100% 

1943 

1787 

1625 

1456 

1280 

1096 

110% 

1832 

1682 

1528 

1368 

1200 

1025 

' 120% 

1730 

1589 

1441 

1287 

1127 

963 

130% 

1642 

1504 

1363 

1216 

1063 

900 

140% 

1561 

1429 

1291 

1152 

1005 

855 

150% 

1486 

1360 

1229 

1094 

955 

811 

160% 

1419 

1296 

1171 

1041 

908 

773 

170% 

1357 

1239 

1118 

992 

855 

180% 

1298 

1186 

1069 

949 

826 

190% 

1246 

1136 

1026 

908 

788 

200% 

1201 

1090 

983 

871 

The  temperatures  are  in  Centigrade  degrees  and  may  be 
changed  to  Fahrenheit  by  multiplying  by  1.8  and  adding  32. 


BURNING  CLAY  WARES. 


67 


One  hundred  per  cent,  of  air  is  the  theoretical  amount  for 
perfect  combustion,  and  for  such  or  greater  amount  of  air  we 
can  be  reasonably  certain  of  the  composition  of  the  combus- 
tion gases  and  of  the  accuracy  of  the  calculated  results  in 
consequence.  Below  100  per  cent,  of  air  there  is  always  un- 
certainty of  the  composition  of  the  gas,  with  corresponding 
doubt  in  the  calculated  temperatures.  The  table  illustrates 
the  fact  that  it  is  possible  to  get  somewhat  higher  tempera- 
tures with  less  than  100  per  cent,  than  with  more  than  100 
per  cent.,  within  a reasonable  variation  from  100  per  cent.  In 
the  second  column  we  get  1861  degrees  with  90  per  cent,  of 
air,  against  1832  degrees  with  110  per  cent.,  and  likewise  70 
per  cent,  is  as  good  or  better  than  130  per  cent.,  when  we 
consider  the  oxygen  taken  from  the  ware  to  satisfy  the  gases, 
and  also  the  undoubted  kiln  wall  leakage.  If  a kiln  is  hanging 
up  and  it  is  evident  that  a higher  temperature  is  required, 
the  proper  course  is  to  reduce  the  draft,  provided  the  draft  is 
free,  and  at  the  same  time  close  up  the  fires  to  shut  off  the 
excess  air. 

It  may  be  that  the  kiln  bottom  is  choked  and  that  the  air 
supply  is  exceedingly  low,  and  there  is  no  help  for  such  a kiln 
condition.  It  should  not  have  occurred. 

Specific  Heat. 

Specific  heat  is  the  heat  required  to  advance  the  tempera- 
ture of  a given  weight  of  any  substance  relative  to  that  re- 
quired to  advance  the  temperature  of  an  equal  weight  of  water 
through  the  same  range  of  temperature,  or  more  properly  it 
is  the  heat,  measured  in  calories  of  B.t.u.,  required  to  ad- 
vance the  temperature  of  a unit  weight  of  any  substance  one 
degree. 

Unfortunately,  for  simple  calculations  the  specific  heat  of 
a gas  is  not  constant,  but  instead  increases  with  the  tempera- 
ture. 

For  instance,  hydrogen  at  atmospheric  temperature  has  a 
specific  heat  of  3.37,  while  at  2000  degrees  F.  the  specific  heat 
is  4.05.  If  we  had  a pound  of  hydrogen  heated  to  2000  degrees 
the  heat  required  to  attain  this  temperature  will  be  neither 
3.37X2000=6740  B.t.u.,  nor 
4.05X2000=8100  B.t.u., 

but  will  be  an  average  between  these  two  values,  or  7420  B.t.u. 

Recent  investigations  of  specific  heats  of  gases  at  high 


68 


BURNING  CLAY  WARES. 


temperatures  Fahrenheit  give  the  following  formulas  for  equal 
weights  under  constant  pressure : 

Nitrogen  0.2405 +.000012  (t— 32) 

Oxygen  0.2104 +.000010  (t— 32) 

Water  vapor  0.42  +.00010  (t — 32) 

Sulphur  dioxide  0.125  +.000056  (t — 32) 

Carbon  dioxide  0.19  +.00006  (t — 32) 

Carbon  monoxide  0.2405 +.000012  (t — 32) 

Hydrogen  3.37  +.00017  (t— 32) 

Methane  0.611  +.00021  ft— 32) 

Air  0.234  +.000012  (t— 32) 

Thermal  calculations  are  much  simplified  by  using  equal 
weights,  and  for  this  reason  we  give  only  specific  heats  for 
equal  weights.  If  the  reader  wishes  to  investigate  thermal 
capacities  of  equal  volumes  he  will  find  the  specific  heat 
formulas  for  such  in  “Coal,”  by  Somermeier;  “Metallurgical 
Calculations,”  by  Richards ; “Industrial  Furnaces,”  by  Da- 
mour,  translation  by  Queneau. 

In  the  use  of  the  above  formulas  if  we  wish  the  specific 
heat  at  any  given  temperature,  we  must  double  the  prefix  of 
(t — 32),  but  in  combustion  problems  we  seldom  have  occasion 
to  determine  such  specific  heats. 

If  we  wish  to  determine  the  heat  in  any  weight  of  gas  from 
atmospheric  temperature  to  a higher  temperature  we  use  the 
formula  above  given  for  the  gas  in  question. 

If  we  wish  to  determine  the  heat  in  a gas  between  any  two 
temperatures,  as  “T”  and  “t,”  we  use  the  value  (T+t)  instead 
of  (t — 32)  in  the  above  formulas,  first  deducting  32  from  both 
“T”  and  “t,”  and  then  multiply  through  by  (T — t). 

For  example,  the  heat  in  nitrogen  between  632  and  1832 
degrees  would  be 

[.2405+. 000012  (2400)  ] 1200=323.16  B.t.u. 

To  convert  the  formulas  to  calories  multiply  the  prefix  of 
(t — 32)  by  1.8.  The  formula  for  the  specific  heat  of  water 
vapor,  in  calories,  will  be  0.42+.00018T,  in  which  “T”  is  in 
centigrade  degrees. 

Theoretically,  in  the  use  of  the  formulas  we  should  always 
take  zero  centigrade  for  the  atmospheric  temperature  and  for 
any  temperature  above  zero  we  should  use  the  values  (T+t) 
and  (T — t)  in  making  thermal  calculations,  but  practically 
we  may  use  the  formulas  as  given  for  any  atmospheric  tem- 
perature below  100  degrees  Fahrenheit  to  any  higher  tem- 
perature, because  the  increment  in  the  specific  heat  between 


BURNING  CLAY  WARES. 


69 


freezing  and  100  degrees  is  inappreciable  and  may  be  neglected. 

With  the  formulas  we  can  calculate  the  heat  value  in  any 
gas  and  the  following  table  has  been  determined  from  them : 


Thermal  Capacities  of  Cases. 


Temp. 

F. 

n2,co 

O 

h2o 

CO2 

H 

ch4 

Air 

so2 

32 

0 

0 

0 

0 

0 

0 

0 

0 

232 

49 

42 

88 

40 

581 

131 

47 

27 

432 

98 

86 

184 

86 

1375 

278 

95 

59 

632 

149 

130 

288 

136 

2083 

442 

145 

95 

832 

200 

175 

400 

190 

2895 

623 

195 

136 

1032 

252 

220 

520 

250 

3540 

821 

246 

181 

1232 

306 

267 

648 

314 

4289 

1035 

298 

231 

1432 

360 

314 

748 

384 

5051 

1266 

351 

285 

1632 

415 

362 

928 

458 

5827 

1515 

405 

343 

1832 

471 

411 

1080 

536 

6617 

1780 

460 

406 

2032 

529 

461 

1240 

620 

7420 

2062 

516 

474 

2232 

587 

511 

1408 

708 

8237 

2361 

573 

546 

2432 

646 

562 

1584 

802 

9067 

2676 

631 

623 

2632 

706 

615 

1768 

900 

9911 

3008 

690 

704 

2832 

767 

668 

1960 

1002 

10769 

3357 

749 

789 

3032 

830 

721 

2160 

1110 

11640 

3723 

810 

879 

Intermediate  temperature  capacities  can  be  found  direct 
from  the  formulas  or  by  interpolation.  Theoretically  the  lat- 
ter does  not  give  correct  values,  but  they  are  near  enough  for 
practical  work,  and  especially  in  view  of  the  fact  that  the 
specific  heats  as  determined  by  different  investigators  are  not 
identical,  and  although  the  difference  is  small,  yet  it  is  greater 
than  any  error  that  would  be  introduced  by  interpolated  values 
within  a range  of  200  degrees. 


Calorific  Determinations. 

The  determination  of  calorific  values  in  kiln  operations  can 
be  best  illustrated  by  a concrete  example.  The  average  analysis 
of  a Hocking  Valley  (Ohio)  coal  as  given  by  Somermeier  in 
“Coal”  is  as  follows : 

Per  Cent.  Pounds. 


H 5.43  .0543 

C 69.03  .6903 

N 1.26  .0126 

O 13.62  .1362 

S 3.30  .0330 

Ash  7.36  .0736 


The  heat  value  in  this  coal  in  B.t.u.,  according  to  Dulong’s 
formula,  is : 

14544 X. 6658 +62028  (.0543— .0158)  +4050X-0297=12192. 

If  we  assume  that  25  per  cent,  of  the  kiln  ash  is  carbon, 


70 


BURNING  CLAY  WARES. 


the  total  ash  will  be  9.81  per  cent.,  of  which  2.45  per  cent,  is 
carbon,  leaving  66.58  per  cent,  carbon  for  combustion. 

If  10  per  cent,  of  the  sulphur  remains  in  the  ash,  we  have 
2.97  per  cent,  remaining  for  combustion. 

The  above  adjustments  give  us  the  following  data : 


H. 

= .0543  pound 

C. 

= .6658  pound 

N. 

= .0126  pound 

O. 

= .1362  pound 

s. 

= .0297  pound 

Ash 

= .1014  pound 

1.0000  pound 

The  oxygen  will  combine  with  the  hydrogen  to  form  water 
vapor  in  the  proportions  of 

2 x 
16  .1362 

from  which  we  determine  x to  be  .017,  making  the  water 
vapor  from  this  source  .1362+.017=.1532.  There  remains 
.0373  H.,  which  will  combine  with  oxygen  from  the  air,  and 
which  will  require  .0373X8=.2984  oxygen,  and  produces,  0.373 
+ .2984=. 3357  water  vapor. 

The  total  water  vapor  from  the  coal  is,  therefore,  .1532+ 
,3357=.4889  pounds. 

The  carbon  burns  to  C02  if  the  combustion  is  complete, 
and  it  will  require  oxygen  from  the  air  in  the  ratio  of 

12  x 

32  .6658 

from  which  we  determine  x to  be  1.7755  and  the  C02  is  2.4413 
pounds.  The  sulphur  burns  to  S02  and  requires  an  equal 
amount  of  air,  or  .0297  pounds,  and  develops  .0594  pounds  S02. 
The  total  oxygen  from  the  air  for  the  several  reactions  is 
2.1036  pounds,  and  this  carries  with  it  7.005  pounds  of  ni- 
trogen. 

We  have,  then,  as  products  of  complete  combustion : 

Water  vapor  .4889  pound 

C02  2.4413  pounds 

S02  0594  pound 

N in  coal 0126  pound 

N from  air 7.0050  pounds 

The  total  air  required  is  9.1086  pounds,  exclusive  of  any 
moisture  in  the  air. 


BURNING  CLAY  WARES. 


71 


In  9.1086  pounds  of  air  at  an  assumed  temperature  of  80 
degrees  F.  and  80  per  cent,  humidity,  there  will  be  approxi- 
mately .16  pounds  of  moisture  (see  Kent,  page  551),  which 
must  be  added  to  the  moisture  from  the  coal,  thus  increasing 
the  water  vapor  to  .6489  pounds. 

In  down-draft  kilns  under  full  fire,  the  temperature  of  the 
gases  is  that  of  a red  heat — let  us  assume  1432  degrees  F. 

From  the  above  data  and  the  table  of  Thermal  Capacities 
we  determine  the  heat  carried  out  of  the  kiln  by  the  waste 
gases  as  follows: 

.6489  X 784=  508.7  B.  t.  u. 

2.4413X384=  937.5  B.  t.  u. 

.0594X285=  16.9  B.  t.  u. 

7.0176X360=2526.3  B.  t.  u. 

3389.4  B.  t.  u. 

We  have  stated  that  the  oxygen  in  the  coal  combines  with 
hydrogen  to  form  water,  but  in  fact  it  undoubtedly  is  already 
in  such  combination,  and  we  must  consider  as  a loss  the  con- 
version of  this  water  into  vapor.  .1532X970=148.6  B.  t.  u., 
the  latent  heat  of  the  vapor,  which,  added  to  the  above,  gives 
a total  stack  loss  of  4138  B.  t.  u.  per  pound  of  coal. 

The  total  heat  value  of  the  available  combustibles  in  the 
fuel  is  12192  B.  t.  u.  and  the  heat  losses  in  the  gases  are  there- 
fore 34  per  cent.  Few  kiln  operations  are  conducted  with  the 
exact  requirement  of  air,  upon  which  basis  the  above  problem 
is  figured.  One  factory  record,  during  a test  of  three  hours, 
shows  an  average  of  216  per  cent,  excess  of  air.  The  excess 
varied  from  25  per  cent,  to  660  per  cent.  These  records  are 
not  unusual,  and  the  average  in  many  plants  will  not  be  less 
than  100  per  cent,  excess  air,  although  it  should  not  exceed  50 
per  cent,  with  proper  furnace  construction  under  reasonably 
good  operation.  In  some  operations  the  air  is  reduced  practi- 
cally to  the  theoretical  requirement,  otherwise  it  would  not  be 
possible  to  burn  some  lines  of  high  temperature  ware. 

In  the  preceding  problem  the  theoretical  amount  of  air  is 
9.1086  pounds,  carrying  .16  pound  of  moisture.  One  hundred 
per  cent,  excess  air  would  carry  3346  B.  t.  u.  out  of  the  stack, 
which  would  be  27  per  cent,  of  the  fuel  value,  making  a total 
loss  of  61  per  cent.,  and  200  per  cent,  excess  air  brings  the  total 
loss  to  88  per  cent.  Bleininger’s  tests  show  an  average  loss 
of  31.7  per  cent,  in  the  fuel  gases.  The  stack  losses  which  we 
show  are  high  because  we  have  assumed  a high  stack  tempera- 
ture, but  we  get  stack  temperatures  approximating  this  degree 


72 


BURNING  CLAY  WARES. 


in  multiple  stack  kilns  where  the  stack  is  built  into  the  kiln 
wall,  and  in  such  kilns  the  heat  loss  by  absorption  will  be  rela- 
tively less. 

If  the  stack  is  some  distance  from  the  kiln  and  the  gases 
are  cooled  to  600  or  800  degrees,  the  stack  loss  will  be  corre- 
spondingly less,  but  the  kiln  absorption  will  be  greater. 

As  has  been  stated,  the  air  entering  the  grates  burns  the 
carbon  to  COa,  and  as  this  rises  through  the  coals  it  is  reduced 
to  CO,  which  burns  to  C02,  when  it  comes  in  contact  with  the 
secondary  air. 

If  the  air  volume  is  insufficient,  the  stack  gases  will  con- 
tain both  CO  and  C02.  In  making  a calculation  for  such  a 
condition,  we  first  estimate  the  volume  of  CO  and  determine 
the  amount  of  air  entering  into  it.  The  remaining  air  supply 
will  convert  part  of  the  CO  into  C02. 

Assuming  that  the  air  supply  is  80  per  cent.,  which  would 
give: 

2.1036  X .80=1.6828  pounds  oxygen. 

7.026  X .80=5.6044  pounds  nitrogen. 

.16  X.80=  .128  pounds  moisture. 

We  must  first  satisfy  the  hydrogen  and  sulphur.  Hydrogen 
requires  8 pounds  of  oxygen  per  pound  of  hydrogen,  and  since 
there  was  .0373  pounds  of  hydrogen  unsatisfied,  we  must  allow 
for  it  .2984  pounds  of  oxygen.  The  .0297  pounds  of  sulphur  re- 
quires .0297  pound  of  oxygen.  The  total  oxygen  for  hydrogen 
and  sulphur  is  .2984 +.0297 =.3281  pound,  leaving,  (1.6828 — 
.3281),  1.3547  p^  s of  oxygen  for  the  carbon.  The  carbon 
first  burns  to  CO,  aich  requires 

3 .6658 


4 x 

from  which  we  find  x to  be  .8877  oxygen,  leaving,  (1.35447— 
.8877),  .467  oxygen  available  to  burn  CO  to  C02.  The  CO 
thus  formed  is  1 :5535  pounds.  The  CO  which  can  be  burned 
by  .467  pound  of  oxygen  will  be  as 

28  x 


16  .467 

from  which  x is  found  to  be  .8173.  Gathering  the  results  to- 
gether, we  have: 


BURNING  CLAY  WARES. 


73 


.0373  H +.2984  0 = .3357  H20 
.0297  S +.0297  0 = .0594  S02 
1.5535  CO  —.8173  CO=  .7362  CO 
.9173  CO +.467  O =1.2843  COa 


Total  = 2.4156  poundfij 
Combustion  products  = 2.4156 
Nitrogen  = 5.6044 

Moisture  from  coal  = .1532 
Moisture  from  air  = .1280 


Total  stack  gases,  8.3012 

The  calorific  determination  is  the  same  as  for  complete  com- 
bustion, and  is  as  follows : 

.6179  H20  X 784  = 484.4  B.t.u. 

.0594  S02  X 285  = 16.9  B.t.u. 

.7362  CO  X 360  = 265.0  B.t.u. 

1.2843  C02  X 384  = 493.2  B.t.u. 

5.6044  N X 360  = 2017.6  B.t.u. 


3277.1  B.t.u. 

Latent  heat  water  vapor  118.9  B.t.u. 

Total  3396  B.t.u. 

This  is  28  per  cent,  of  the  total  fuel  value  of  the  coal. 

Less  than  the  theoretical  per  cent,  of  air  reduces  the  stack 
losses,  but  on  the  other  hand  we  do  not  get  the  full  heat  value 
of  the  fuel.  We  have  .7362  pond  of  unburned  CO,  which  would 
give  3209  B.  t.  u.,  about  23  per  cent,  of  which  is  offset  in  the 
lessened  stack  loss.  As  has  been  previously  pointed  out,  if  we 
wish  high  temperatures,  regardless  of  the  reducing  effect  of  the 
kiln  gases,  it  is  better  to  err  on  the  side  of  less  than  the  theo- 
retical amount  of  air  than  on  the  side  of  excess  air  up  to  a 
difference  of  20  to  30  per  cent,  from  the  theoretical  amount. 

The  stack  gases  from  continuous  kilns  which  escape  at 
temperatures  under  400  degrees  involve  relatively  small  losses 
and  it  has  been  common  practice  to  use  a large  excess  of  air 
in  such  kiln. 

Originally,  continuous  kilns  had  no  advanced  heating  flues 
for  the  water-smoking,  and,  in  view  of  the  small  quantity  of 
fuel  burned  and  in  consequence  the  relatively  small  volume  of 
combustion  gases  in  connection  with  the  low  final  tempera- 
ture, it  was  necessary  to  introduce  an  excess  of  air  in  order  to 
remove  the  moisture  and  thus  keep  the  water-smoking  compart- 


74 


BURNING  CLAY  WORES. 


ments  in  step  with  the  burning  compartment.  Modern  continu- 
ous kilns  are  provided  with  advance  heating  flues  controlled 
by  an  independent  stack  or  fan,  and  there  is  no  reason  why 
any  great  excess  of  air  need  be  carried  through  the  combustion 
compartments. 

Carbon-Dioxide  in  Combustion  Gases. 

The  volumes  of  oxygen  and  nitrogen  in  the  air  are  very 
closely  21  per  cent,  of  the  former  and  79  per  cent,  of  the  latter. 

If  we  were  burning  pure  carbon  with  the  theoretical  amount 
of  air,  the  stack  gases  would  contain  21  per  cent,  volume  carbon- 
dioxide  and  79  per  cent,  volume  nitrogen,  but  in  the  combustion 
gases  from  impure  coals  the  carbon-dioxide  is  always  less  than 
21  per  cent.  For  example,  in  perfect  combustion  of  the  coal  in 
question,  the  C02  in  the  combustion  gases  would  be  16.4  per 
cent.,  including  the  moisture  shown,  or  including  only  the 
amount  of  moisture  which  would  be  present  in  a sample  of  the 
gas  at  62  degrees  F.,  the  C02  would  be  17.7  per  cent.  Any  gases 
having  materially  less  C02  and  at  the  same  time  free  oxygen  is 
evidence  of  excess  air,  and  the  amount  of  the  excess  may  be 
readily  calculated.  If  the  gases  show  C02  and  CO  with  no 
oxygen,  or  only  a trace,  the  combustion  is  not  complete  through 
lack  of  air. 

Under  the  assumption  of  200  per  cent,  excess  of  air  there 
will  be  5.6  per  cent,  of  C02  in  the  gases. 

The  following  table  shows  the  percentage  of  carbon-dioxide 


which  the  kiln  gases  from  an 
the  various  percentages  of  air: 

Illinois  coal 

will  contain  under 

Air. 

C02 

Air. 

C02 

100% 

15.9 

160% 

10.2 

no% 

14.5 

170% 

9.6 

120% 

13.4 

180% 

9.1 

130% 

12.4 

190% 

8.6 

140% 

11.6 

200% 

8.2 

150% 

10.8 

300% 

5.5 

Under  certain 

conditions  of 

temperature  and  pressure  (32 

degrees  F.  and  29.92  inches  pressure)  the  volumes  of  perfect 
gases  are  inversely  proportional  to  the  molecular  weights  and 
the  imperfect  gases,  particularly  C02,  may  also  be  included, 
because  the  error  introduced  in  so  considering  them  is  slight 
in  comparison  with  the  errors  in  an  ordinary  gas  analysis. 

(Note: — The  analytical  work  as  usually  conducted  in  clay- 
working factories  is  only  an  approximation  of  the  actual  com- 
position of  the  kiln  gases.  The  average  operator  who  sticks 


BURNING  CLAY  WARES. 


75 


an  iron  pipe  into  a stack  and  collects  the  gas  over  water,  who 
is  careless  in  connecting  the  apparatus  and  who  pays  no  atten- 
tion to  the  temperature  and  pressure  changes,  is  only  making  a 
good  guess  of  the  composition  of  the  gas,  but  if  the  combustion 
is  widely  varying  from  perfect  combustion,  a good  guess  will 
lead  to  more  economical  operation,  and  is  therefore  recom- 
mended.) 

Two  unit  weights  of  hydrogen,  28  of  nitrogen,  28  of  car- 
bon-monoxide, 44  (approximately)  of  carbon-dioxide,  64  (ap- 
proximately) of  sulphur-dioxide,  18  of  water  vapor,  16  of 
methane,  28  of  ethylene,  have  the  same  unit  volume.  Having 
the  weights  of  the  combustion  gases,  we  determine  the  relative 
volumes  by  dividing  the  molecular  weights,  and  from  these 
we  get  the  per  cent,  volumes,  such  as  would  be  determined 
in  a gas  analysis  apparatus.  In  this  way  we  get  the  16.4  per 
cent,  carbon-dioxide  in  the  combustion  gases  under  considera- 
tion, and  similarly  were  the  results  in  the  above  determined. 


Radiation  Losses. 

The  radiation  losses  are  much  higher  than  the  average 
clay  worker  suspects,  and  if  he  could  actually  see  the  loss 
going  on,  he  would  take  steps  to  reduce  it  to  a minimum.  He 
sees  the  coal  being  shoveled  into  the  furnace  and  the  dwin- 
dling coal  pile,  but  these  are  necessary  factors  in  burning  the 
ware. 

Bleininger’s  tests  of  industrial  kilns  show  an  average  of 
53.7  per  cent,  of  the  fuel  value,  taken  up  by  the  kiln  and  lost 
by  radiation.  Unfortunately,  there  is  no  data  relative  to  the 
actual  radiation  loss,  and  we  can  only  approximate  it  by  an 
assumption  of  the  heat  taken  up  by  the  kiln.  An  average 
kiln  will  have  in  its  walls  as  many  tons  of  brick  work  as  the 
tonnage  capacity  of  the  kiln,  if  filled  with  bricks,  and  the 
average  heat  taken  up  by  the  walls  would  be  roughly  one- 
half,  since  the  outer  surface  of  the  kiln  walls  are  scarcely 
hotter  than  the  atmosphere,  while  the  inner  walls  have  the 
kiln  temperature.  Two  brick  kilns  tested  by  Bleininger  have 
an  average  of  15.4  per  cent,  of  heat  taken  up  by  the  ware,  and 
on  the  above  assumption  the  kiln  walls  will  take  up  less  than 
8 per  cent. 

We  realize  that  the  data  is  very  insufficient,  but  it  points 
to  the  probability  that  the  average  radiation  and  conduction 
kiln  loss  is  at  least  40  per  cent.,  except  when  the  fires  are  be- 


76 


BURNING  CLAY  WARES. 


ing  crowded  to  obtain  final  temperatures,  during  which  period 
a maximum  quantity  of  fuel  is  being  burned. 

Kiln  Temperatures. 

In  the  preceding  problem  we  had  12192  B.  t.  u.  per  pound 
of  coal  after  eliminating  the  carbon  in  the  ash. 

In  determining  kiln  temperatures  we  will  assume  50  per 
cent,  excess  air  which  represents  fairly  good  yard  practice. 
We  will  also  assume  20  per  cent,  radiation  loss  which  would 
apply  to  the  maximum  fuel  firing  state. 

Under  perfect  combustion  we  had  4138  B.  t.  u.  stack  losses 
and  to  this  we  must  add  the  heat  taken  out  by  the  excess  air. 

The  air  requirement  was  9.1086  pounds  and  half  of  this 
will  be  4.5544  pounds,  which  at  1432  degrees  has  a thermal 
capacity  of  1598.5  B.  t.  u.  This  added  to  4138  gives  a total 
of  5736.5  B.  t.  u.  The  radiation  loss  (12192  X .20)  is  2438.4 
B.  t.  u.,  making  a grand  total  of  8174  B.  t.  u.  for  the  stack 
losses  and  radiation. 

We  have  available  for  burning  the  ware  12192  — 8174.9  = 
4017.1  B.  t.  u.  per  pound  of  coal. 

The  problem  of  kiln  temperatures  can  best  be  solved  by 
not  considering  the  stack  losses.  We  have  then  12192  — 
2438.4  = 9753.6  B.  t.  u.  available  for  kiln  temperature,  and  we 
can  determine  the  kiln  temperature  by  a series  of  approxi- 
mations. 

Let  us  assume  a kiln  temperature  of  2432  degrees. 

The  heat  required  to  maintain  the  gases  at  this  temperature 
will  be  as  follows : 

.6489  X 1584  = 1027.9  B.  t.  u. 

2.4413  X 802  = 1957.9  B.  t.  u. 

.0594  X 623  = 37.0  B.  t.  u. 

7.0176  X 646  = 4533.4  B.  t.  u. 

4.5543  X 562  = 2873.8  B.  t.  u. 

10430.0  B.  t.  u. 

Evidently  2432  degrees  are  too  high.  A similar  determina- 
tion of  2232  degrees  gives  us  9403.4  B.  t.  u.,  which  is  less  than 
the  available  heat  and  the  actual  temperature  will  be  between 
2432  and  2232  degrees  and  can  be  readily  determined  as  fol- 
lows : 

10430  — 9403.4  = 1026.6  B.  t.  u.  Difference  for  200  degrees. 

9753.6  — 9403.4  = 350.2  B.  t.  u.  Actual  difference. 

200  X 

— = from  which  we  find  X to  be  68  degrees 


1026.6  350.2 


BURNING  CLAY  WARES. 


77 


2232  + 68  = 2300  degrees,  which  is  the  possible  kiln  tempera- 
ture. 

If  the  air  enters  at  62  degrees  it  adds  89  degrees  to  the 
possible  temperature,  making  a total  temperature  of  2398  de- 
grees or  cone  9. 


Kiln  Temperature  from  Coal  and  Producer  Gas. 

On  a preceding  page  we  gave  a table  of  possible  kiln  tem- 
peratures from  an  Illinois  coal  under  different  radiation  losses 
and  percentages  of  air.  In  Vol.  XII,  Trans.  A.  C.  S.  is  a 
paper  by  T.  W.  Garve  and  the  author,  showing  the  method  of 
determining  kiln  temperatures  for  coal,  lignite  and  producer 
gas  under  varying  conditions.  We  will  not  attempt  here  to 
duplicate  this  work,  but  in  addition  to  the  table  already  given, 
the  following  values  have  been  worked  out  for  a number  of 
coals.  The  assumed  conditions  are,  excess  air,  25  per  cent, 
and  radiation  loss,  20  per  cent.  The  analytical  data  is  taken 
from  U.  S.  Geological  Survey,  Professional  Paper  No.  48,  “Re- 
port on  Coal  Tests.” 


Moisture. 

Temperature. 

Coal. 

Per  cent. 

F.  Degrees 

Alabama  No.  2 

3.8 

2603 

Colorado  No.  1 

20.26 

2482 

Illinois  No.  3 

7.6 

2597 

Illinois  No.  4 

12.5 

2561 

Indiana  No|  1 

11.3 

2570 

Oklahoma  No.  1 

4.6 

2594 

Iowa  No.  2 

16.5 

2556 

Kansas  No.  5 

4.3 

2628 

Kentucky  No.  3 

7.08 

2585 

Missouri  No.  2 

11.58 

2561 

Montana  No.  1 

11.49 

2549 

No.  Dakota  No.  2..., 

39.28 

2344 

W.  Virginia  No.  1. . . , 

1.4 

2633 

W.  Virginia  No.  8 

2.57 

2624 

W.  Virginia  No.  12. . . 

1.54 

2650 

Wyoming  No.  2 

9.53 

2554 

Texas  No.  2 

33.7 

2363 

In  actual  average  practice  we 

will  readily  lower  tempera- 

tures  from  the  coal 

because  of 

greater  excess  of  air  than 

estimated,  and  also  because  of  greater  radiation  loss. 


Fuel  Oil. 

In  the  preceding  discussion  of  heat  values  we  have  con- 
fined ourselves  chiefly  to  coal,  and  giving  other  fuels  no  con- 
sideration. Crude  and  fuel  oils  have  been  used  extensively 


78 


BURNING  CLAY  WARES. 


in  burning  clay  wares,  but  the  high  price  of  oil  has  put  an 
end  to  its  use  except  in  California,  and  in  a few  factories  in 
the  East,  where  the  values  of  the  ware  will  stand  a high  fuel 
cost,  or  where  the  oil  consumption  is  very  small,  as  in  Chicago. 

California,  with  its  large  output  of  oil,  with  an  asphaltic 
base,  and  in  consequence  unfit  for  direct  use  in  engines  and 
without  any  other  fuel  at  a reasonable  cost,  is  naturally  the 
field  where  oil  will  be  used  for  a long  time. 

Every  user  of  oil  fuel  has  experienced  initial  troubles, 
chief  of  which  is  the  difficulty  of  getting  the  heat  from  the 
furnace  or  burner  over  into  the  kiln,  with  the  result  that  the 
furnace  arches  and  throats  were  melted  down.  The  oil  is 
brought  to  the  kiln  under  pressure  and  sprayed  into  the  fur- 
nace and  atomized  by  steam  or  air  under  pressure.  Whether 
steam  or  air  is  the  better  medium  is  a matter  of  opinion,  but 
it  is  our  opinion  that  the  advantage  rests  with  air.  The 
advocates  of  steam  say  that  the  steam  heats  the  oil  in  its 
passage  through  the  burner  and  gives  better  atomization, 
that  it  aids  in  taking  the  heat  from  the  furnace  into  the  kiln, 
and  finally  that  it  is  simpler  in  its  application.  Steam,  how- 
ever, does  not  support  combustion  and  the  air  supply  is  de- 
pendent upon  the  kiln  draft  and  the  aspirating  effect  of  the 
spray  of  oil  and  steam.  Some  of  the  steam  may  be  converted 
into  water-gas,  which,  as  has  been  stated,  serves  a good  pur- 
pose as  a carrier  of  heat,  but  water  vapor  has  a thermal 
capacity  double  that  of  air  or  of  the  gaseous  product  of  com- 
bustion. Steam  is  troublesome  to  use  for  low  temperature 
fires  on  account  of  excessive  carbonization  clogging  the 
burners,  but  the  chief  fault  is  that  such  a large  volume  of 
steam  causes  excessive  condensation  in  the  cooler  parts  of 
the  kiln.  The  introduction  of  secondary  air  necessary  for 
complete  combustion  has  been  a problem  where  steam  was 
used,  and  in  a number  of  instances  this  has  been  worked  out 
by  building  a double  furnace,  or  constructing  an  air  flue 
under  the  furnace  floor  extending  to  the  back  of  the  com- 
bustion chamber.  Many  old  plants  changing  from  coal  to  oil 
simply  paved  the  grate  bars  with  bricks  and  thus  had  the 
double  construction  which  has  proven  to  give  better  combus- 
tion results  than  a simple  fire-box  depending  entirely  upon 
draft  and  aspiration  for  air  supply. 

Everyone,  too,  has  experimented  with  flash  walls  or  some 
obstruction  in  the  furnace  to  distribute  the  atomized  oil  in 
the  combustion  chamber  and  mix  it  with  the  air  supply.  The 


BURNING  CLAY  WARES. 


79 


flash  wall  is  desirable  in  the  early  stages  of  the  burning,  but 
when  the  furnace  has  reached  incandescence  no  flash  wall  is 
necessary. 

Air  has  the  advantage  that  it  serves  for  combustion  and 
there  is  much  better  control  of  the  kiln  atmosphere.  Careful 
tests  have  shown  that  there  is  some  economy  in  fuel  in  favor 
of  air  against  steam,  but  the  difference  is  not  sufficient  to 
warrant  a change  from  steam  to  air  simply  for  this  economy. 
The  air  pressure  is  usually  20  pounds  to  30  pounds,  but  there 
are  some  advocates  of  higher  and  others  of  lower  pressure. 

The  early  installations  put  the  oil  supply  above  the  kiln 
level  and  brought  the  oil  to  the  kiln  by  gravity,  but  the  insur- 
ance companies  put  a stop  to  this.  Now,  the  storage  tanks 
for  oil  are  under  ground  and  the  oil  is  either  pumped  direct 
to  the  kiln  or  is  pumped  into  a standpipe,  in  which  a con- 
stant head  is  maintained  by  means  of  an  overflow  back  to 
the  storage  tank. 

The  standpipe  is  by  far  the  better  method,  because  the 
pressure  is  absolutely  constant,  the  pulsations  of  the  pump 
noticeable  in  the  fires  where  the  pump  service  is  direct  are 
eliminated,  and  no  exact  adjustment  of  the  pump  is  required. 

There  are  a number  of  patented  burners  on  the  market, 
and  many  home-made  burners  in  use.  There  are  three  basic 
principles  and  all  burners  are  constructed  on  one  or  the  other 
of  these  principles.  In  general,  there  are  two  pipes,  one  for 
oil  and  one  for  air  or  steam  and  the  nozzle  of  these  pipes  are 
so  placed  that  the  oil  is  sprayed  into  the  combustion  chamber. 
One  pipe  may  be  inside  the  other,  the  inner  tube  for  oil  and 
the  outer  tube  for  air  or  vice  versa,  or  the  two  pipes  may  be 
side  by  side. 

The  atomization  may  take  place  inside  the  outer  tube 
where  the  tubes  are  concentric,  or  at  its  nozzle,  or  the  spray- 
ing may  be  entirely  outside  the  tubes  on  the  principle  of  a 
sand-blast  where  the  material  to  be  sprayed  drops  into  the 
horizontal  blast  of  the  spraying  medium.  One  important  point 
in  any  burner  is  that  the  oil-control  valve  should  be  easily 
removable  so  that  the  nozzle  can  be  cleared  of  any  material 
clogging  it. 

The  advantages  of  oil  fuel  are: 

(1)  Economy  in  labor. 

(2)  Saving  in  storage  space. 

(3)  Elimination  of  hauling  of  coal  and  ash. 


80 


BURNING  CLAY  WARES. 


(4)  No  cleaning  of  furnaces. 

(5)  More  compact  yard  arrangement. 

(6)  Close  control  of  the  combustion  and  kiln  atmosphere. 

(7)  Quicker  burning,  which  means  greater  capacity  or 

fewer  kilns. 

(8)  Cleaner  ware. 

The  disadvantage,  one  and  only,  is  the  prohibitive  cost  of 
the  oil. 

The  heating  value  of  any  oil  may  be  determined  from  an 
analysis  by  Damour’s  method,  but  the  oils  average  so  closely 
to  19000  B.  t.  u.  that  any  calculation  of  their  value  is  not 
necessary.  The  thermal  capacities  of  the  products  of  com- 
bustion enable  us  to  determine  the  stack  losses  in  the  same 
manner  as  for  coal,  but  it  is  not  of  sufficient  importance  to 
repeat  the  calculations  for  this  fuel. 


BURNING  CLAY  WARES. 


81 


CHAPTER  V. 


PRODUCER  GAS, 


HE  INCREASING  tendency  to  use  producer  gas  in  the 


clayworking  industries,  the  comparatively  recent  prac- 


tical application  of  it  to  periodic  kilns,  the  diminishing 
supply  of  natural  gas,  and  the  prohibitive  cost  of  oil,  make  a 
discussion  of  this  new-old  fuel  a pertinent  one. 

A producer  in  its  elementary  form  is  simply  a deep  fire- 
box with  a grate  bottom  and  a closed  top.  The  coal  is  dumped 
into  the  box  from  above  and  burns  on  the  grates  where  the 
ash  accumulates.  The  draft  may  be  induced  or  forced,  and 
accordingly  we  have  suction  and  pressure  producers.  The 
pressure  producer  is  the  type  used  in  clayworking  factories 
and  steam  is  introduced  with  the  air.  We  will  limit  our  dis- 
cussion to  this  type  of  producer. 

The  fuel  bed  may  be  divided  into  four  zones — ash,  com- 
bustion, dissociation  and  distillation.  The  oxygen  in  the  air 
entering  through  the  grates  and  ash  converts  the  incandescent 
carbon  in  the  combustion  zone  into  carbon-dioxide  (C02).  If 
the  fuel  bed  is  shallow,  the  gas  escaping  from  the  top  of  the 
fuel  bed  is  largely  carbon-dioxide  and  nitrogen,  but  if  the  bed 
is  deep,  the  carbon-dioxide,  after  the  air  is  exhausted,  gives 
up  one  molecule  of  oxygen  to  the  incandescent  carbon.  It 
would  be  more  proper  to  say  that  the  carbon  will  take  oxygen 
from  the  carbon-dioxide  resulting  in  two  molecule  of  carbon- 
monoxide  (CO),  which  is  the  chief  combustible  in  producer 
gas.  This  dissociation  of  carbon-dioxide  takes  place  at  a low 
red  heat. 

Water  vapor  passing  over  incandescent  carbon  is  also  dis- 
sociated, resulting  in  hydrogen  and  carbon-dioxide  at  low  tem- 
peratures or  hydrogen  and  carbon-monoxide  at  high  tempera- 
tures. Carbon-monoxide  cracks  at  low  temperatures,  forming 
carbon-dioxide  and  carbon,  but  this  reaction  decreases  with 
increasing  temperature  and  around  1800°  F.  does  not  occur 
at  all. 


82 


BURNING  CLAY  WARES. 


The  volatile  bases  in  the  fuel  are  driven  off  and  dissociated 
in  the  distillation  zone  and  the  resulting  products  pass  into 
the  gas  flues.  The  heavy  hydrocarbons  by  reaction  with  the 
moisture  and  carbon  are  dissociated  into  hydrogen,  carbon- 
monoxide,  some  carbon-dioxide,  and  methane  (CH4)  in  the 
series  of  lighter  gases,  with  the  heavy  hydro-carbon  (tar) 
products  at  the  other  end  of  the  series.  The  higher  the  tem- 
perature at  which  the  distillation  occurs,  the  greater  the  vol- 
ume of  hydrogen  and  carbon-monoxide,  while  at  low  tempera- 
ture there  is  an  increase  in  hydro-carbon  gases  and  a greater 
quantity  of  heavier  tar  product.  In  view  of  the  several  reac- 
tions which  occur  in  order  to  produce  a profitable  producer 
gas,  it  becomes  evident  that  several  important  factors  must 
have  the  maximum  efficiency  value. 

(1)  The  character  of  the  coal. 

(2)  The  depth  of  the  fuel  bed. 

(3)  The  character  of  the  ash. 

(4)  The  volume  of  steam. 

(5)  Temperatures  in  the  producer  and  of  the  producer  gas. 

(6)  Operation  of  the  producer. 

(7)  The  size  and  construction  of  the  producer. 

Anthracite  coal  and  coke  are  the  best  fuels  because  they 
are  practically  free  from  volatile  hydro-carbons,  from  which 
soot  and  tar  are  developed,  and  the  resulting  gas  is  carbon- 
monoxide,  carbon-dioxide  and  hydrogen.  These  fuels  are  sized 
or  may  readily  be  sized,  are  free  from  dust  and  do  not  swell 
in  burning.  Dry,  non-caking  bituminous  coals  are  better  than 
the  caking  coals,  in  that  they  may  contain  less  tar  products, 
and  also  in  that  they  do  not  swell,  which  tends  to  close  up 
the  draft  spaces.  The  swelling,  however,  is  not  so  serious 
because  it  can  be  controlled  by  the  frequency  of  the  firing 
and  stoking. 

The  depth  of  the  fuel  bed  is  a very  important  factor  and 
it  varies  widely  with  the  character  of  the  coal.  It  is  obvious 
that  a shallow  bed,  such  as  is  essential  in  flat  grate  firing, 
would  defeat  the  purpose  of  the  producer.  The  purpose  of 
flat  grate  firing  is  to  secure  a maximum  degree  of  complete 
combustion  in  the  first  instance,  together  with  a maximum 
temperature.  The  conversion  of  carbon  into  carbon-dioxide 
is  a heat-producing  reaction,  while  the  dissociation  of  carbon- 
dioxide  to  carbon-monoxide  is  a heat-absorbing  reaction,  and 
in  consequence  flat  grate  firing  should  be  limited,  as  far  as 


BURNING  CLAY  WARES. 


83 


practicable,  to  a combustion  zone.  This  gives  a minimum  of 
carbon-monoxide  and  a maximum  temperature.  The  high  tem- 
perature is  essential  in  the  distillation  of  the  hydro-carbons 
and  in  their  dissociation  into  the  lighter  gases,  and  also 
prevents  the  dissociation  of  carbon-monoxide  into  dioxide. 
The  more  complete  the  initial  combustion,  the  less  secondary 
air  will  be  required,  and  in  consequence  there  will  be  less 
cooling  of  the  combustion  gases  and  its  attendant  evil  effects. 
We  do  not  wish  to  give  the  impression  that  carbon-monoxide 
is  not  present  in  a grate  combustion.  Carbon-dioxide  is 
strongly  oxydizing  at  high  temperatures  and  in  contact  with 
the  incandescent  coal  some  of  it  will  be  reduced  to  carbon- 
monoxide,  even  though  there  is  an  excess  of  free  oxygen,  but 
the  development  of  carbon-dioxide  will  be  greatly  in  excess. 

The  bed  of  glowing  coals  in  the  producer  must  be  of  suffi- 
cient depth  to  insure  complete  absorption  of  the  oxygen  from 
the  entering  air,  and  an  additional  depth  to  produce  the  dis- 
sociation of  the  carbon-dioxide  and  water  vapor.  It  is  mislead- 
ing to  separate  the  incandescent  fuel  bed  into  zones,  because 
there  is  no  division  line.  In  the  bottom,  because  of  the  maxi- 
mum oxygen  supply,  there  is  a maximum  degree  of  complete 
combustion.  As  the  free  oxygen  diminishes,  the  carbon, 
greedy  for  oxygen,  begins  to  feed  upon  the  carbon-dioxide. 
Perfect  operation  would  be  complete  absorption  of  the  oxygen 
and  complete  dissociation  of  the  carbon-dioxide  and  water 
vapor.  The  dissociation  reactions,  as  above  stated,  are  heat 
absorbing.  For  example,  a pound  of  carbon  to  carbon-dioxide 
develops  14,544  B.  t.  u.  and  generates  3%  pounds  of  the  gas. 
The  reduction  of  this  gas  to  carbon-monoxide  consumes 
10,170  B.  t.  u. 

The  size  of  the  lumps  of  fuel  materially  affect  the  depth 
of  the  bed  and  the  reactions.  If  large  lumps  with  no  fine  ma- 
terial are  used,  little  resistance  is  offered  to  the  passage  of 
the  gases  and  the  essential  surface  contact  of  the  air  and 
gases  with  the  fuel  is  limited.  There  must,  therefore,  be  a 
deeper  fuel  bed  to  insure  complete  reactions.  The  finer  the 
material,  the  shallower  the  bed.  Very  dusty  material,  which 
would  involve  a correspondingly  shallow  bed,  is  objectionable 
in  that  a higher  pressure  is  necessary,  which  increases  the 
tendency  to  cut  through  the  bed  in  spots.  It  is  hardly  neces- 
sary to  mention  the  fact  that  such  an  operation  would  result 
in  a poor  gas — high  in  carbon-dioxide  in  the  hot  spots  and 
high  in  heavy  hydro-carbons  and  tar  over  the  larger  cool  areas. 
A mixture  of  lump  coal  and  slack  gives  trouble,  in  that  the 


84 


BURNING  CLAY  WARES. 


lumps  roll  to  the  sides  or  center  and  form  chimneys,  while 
the  slack  blankets  the  area  in  which  it  accumulates.  Nagel, 
in  “Producer  Gas  Fired  Furnaces,”  gives  the  following  as  the 
proper  depth  for  several  fuels:  “One-inch  coke,  30  inches; 

l^-inch  coke,  45  inches;  2 *4 -inch  coke,  72  inches;  %-inch 
bituminous  coal,  22  inches;  mine  run  coal,  60  inches  to  80 
inches.  Dusty  fuels  require  less  height  than  coarse,  loose 
fuels.”  It  must  be  noted  that  no  set  rule  can  be  given  for 
the  depth  of  the  fuel  bed.  It  is  determined  by  the  character 
of  the  fuel  and  the  pressure.  The  depth  given  by  Nagel  for 
mine  run  coal  would  be  excessive  for  some  mine  run  coals, 
which  are  notably  dirty. 

The  character  of  the  ash  is  important  in  the  selection  of 
a producer  coal.  An  ash  that  clinkers  badly  builds  up  scaf- 
folds around  the  sides,  gradually  reducing  the  area  to  a small 
pot  in  the  center.  In  one  instance  the  clinkers  were  so 
troublesome  that  it  was  necessary  to  have  an  extra  producer, 
and  each  producer  could  operate  continuously  only  about  forty- 
eight  hours. 

Steam  has  an  important  part  in  producer  gas  and  in  the 
operation  of  the  producer.  It,  with  the  air,  cools  the  ash  of 
the  coal,  and  in  this  work  it  is  nearly  double  the  efficiency  of 
air  on  account  of  its  higher  specific  heat.  The  heat  thus  taken 
from  the  ash  is  returned  to  the  combustion  zone.  Besides  the 
cooling  effect,  the  steam  tends  to  disintegrate  the  weaker 
clinkers,  and  if  their  formation  is  not  entirely  prevented, 
their  structure  is  so  weakened  that  they  are  easily  broken  up 
in  removing  the  ash  from  the  pit.  Water  vapor  is  dissociated 
in  contact  with  glowing  carbon,  forming  hydrogen  and  carbon- 
monoxide.  This  dissociation  is  practically  nil  at  temperatures 
below  1,200°  F.,  but  increases  with  advancing  temperature. 
At  1,800°  F.  the  dissociation  is  above  90  per  cent.,  and  about 
2,200°  F.  is  practically  complete. 

The  dissociation  of  water  vapor  is  a heat-absorbing  reac- 
tion. A pound  of  steam  contains  1/9  pound  hydrogen  and 
8/9  pound  oxygen.  Combustion  of  hydrogen  develops  62,028 
B.  t.  u.,  and  1/9  pound  would  give  6,892  B.  t.  u.  Conversely, 
the  dissociation  of  one  pound  of  water  vapor  will  absorb  6,892 
B.  t.  u.  In  the  dissociation  there  is  set  free  8/9  pound  of 
oxygen,  which  combines  with  the  carbon  to  form  carbon- 
monoxide.  A pound  of  monoxide  develops  4,374  B.  t.  u.  and 
8/9  pound  will  deliver  3,888  B.  t.  u.  The  net  cooling  effect 
of  the  water  vapor  is,  therefore,  2,904  B.  t.  u.  per  pound  of 
vapor. 


BURNING  CLAY  WARES. 


85 


The  absorption  of  heat  by  the  dissociation  of  carbon- 
dioxide  has  been  mentioned,  and  the  cooling  effect  of  this 
reaction  is  in  addition  to  the  cooling  effect  of  the  water  vapor, 
and  it  is  further  noted  that  to  insure  complete  dissociation  of 
the  dioxide  and  the  maintenance  of  the  monoxide  and  also 
that  the  dissociation  of  the  water  vapor  may  be  complete, 
temperatures  around  2,000°  F.  must  be  maintained  in  the  dis- 
sociation zone.  The  initial  temperature  of  the  gases  rising 
from  the  combustion  zone,  including  radiation  and  conduc- 
tion, together  with  the  heat  of  combustion  taking  place  in  the 
dissociation  zone,  must  be  sufficient  to  maintain  the  required 
temperature  in  the  latter  zone.  This,  in  a measure,  deter- 
mines the  amount  of  water  vapor  which  may  be  introduced. 
If  we  could  use  steam  entirely,  we  would  get  a gas  consisting 
of  hydrogen  and  carbon-monoxide — a 100  per  cent,  combustible 
gas,  but  water  vapor  will  not  support  combustion,  and  we 
must  introduce  air  to  develop  and  maintain  the  necessary 
temperatures  in  the  producer. 

Air  contains  by  weight  23  parts  oxygen  and  77  parts  nitro- 
gen. A pound  of  air,  then,  will  contain  23/100  pound  of  oxy- 
gen, which,  in  combination  with  carbon  and  burned  to  the 
final  resultant  monoxide  gas,  will  deliver  1,006  B.  t.  u.  The 
nitrogen  which  accompanies  the  oxygen  is  simply  a dead  load 
which  must  be  heated* up  and  carried  through  the  various 
stages  until  the  waste  gases  are  turned  back  into  the  atmos- 
jjhere,  and  the  heat  which  it  retains  is  a net  loss.  Instead  of 
a 100  per  cent,  combustible  gas,  the  average  producer  gas 
varies  between  30  per  cent,  and  40  per  cent,  combustible. 

We  must  use  enough  air  to  provide  for  all  the  producer 
losses,  including  heating  up  and  dissociating  the  steam.  If 
an  excessive  amount  of  water  is  used,  the  temperature  is 
lowered  and  in  consequence  carbon-dioxide — a non-combusti- 
ble— is  formed,  displacing  the  monoxide — a combustible — and 
also  some  of  the  vapor  will  pass  through  unchanged.  The 
latter  can  be  removed  by  condensation  if  the  gases  are  cooled, 
but  unless  the  initial  heat  is  returned  to  the  producer,  the 
fuel  loss  chargeable  to  the  producer  is  increased.  Carbon- 
dioxide,  on  the  other  hand,  cannot  be  removed  and  it  is  simply 
a gas  burden  which  must  be  carried  to  the  end.  The  weight 
of  steam  used  varies  from  20  per  cent,  to  60  per  cent,  of  the 
weight  of  the  coal  gasified,  depending  upon  the  character  of 
gas  desired,  character  of  the  coal  and  fusibility  of  the  ash. 
Average  practice  would  be  nearly  one  boiler  horsepower  per 
ton  of  coal  per  twenty-four  hours,  which  would  approximate 


86 


BURNING  CLAY  WARES. 


33  per  cent.  Mechanically  operated  producers  require  two- 
thirds  of  this  amount,  or  16  to  20  pounds  of  steam  per  ton 
of  coal  in  twenty-four  hours. 

In  the  analysis  of  Ohio  coal,  given  under  combustion  of 
coal,  we  had  .69  pound  of  carbon.  Thirty-three  per  cent,  of 
the  weight  of  the  coal  would  be  .33  pound  of  steam  per  pound 
of  coal.  From  this  data  we  can  determine  the  relation  of 
steam  and  air.  The  oxygen  in  the  steam  will  be  in  the  pro- 
portion of 

18  .33 

16  X 

from  which  we  find  X = oxygen  = .29  pound.  The  carbon 
taken  up  by  this  oxygen  will  be 

.29  X 
16  12 

X = .22  pound.  This  from  .69  leaves  .47  pound  of  carbon  to 
be  burned  by  the  air  in  the  ratio  of 

.47  X 
12  16  * 

and  X = oxygen  from  air  = .62  pound. 

The  ratio  of  oxygen  to  air  is 

33  .62 

100  X 

and  X = 2.7  pounds  of  air. 

The  weights  of  steam  and  air  reduced  to  percentages  are 
11  per  cent,  of  steam  and  89  per  cent,  of  air.  Similarly,  20 
percent,  of  the  weight  of  the  fuel  in  steam  would  give  8 per 
cent,  of  steam  and  92  per  cent,  of  air. 

Increased  quantity  of  steam  results  in  a higher  hydrogen 
content  in  the  gas,  but  the  cooling  effect  of  the  steam  lowers 
the  dissociation  efficiency,  and  in  consequence  there  is  an  in- 
crease in  the  inert  carbon-dioxide.  Every  fuel  in  producer  use 
is  a separate  problem,  and  the  frequency  of  firing,  depth  of 
fuel  bed,  volumes  of  steam  and  air  and  pressure  must  be 
worked  out  for  each  fuel. 

Steam  does  not  add  any  fuel  value  to  the  gas,  nor  is  the 
ratio  of  nitrogen  and  carbon-dioxide  in  the  waste  stack  gases 


BURNING  CLAY  WARES. 


87 


in  any  way  changed  as  a gas  analysis  will  show.  If  a pro- 
ducer is  blown  with  air  in  large  volume  under  high  pressure, 
we  would  have  the  condition  of  a blast  furnace — high  tem- 
perature, excessive  radiation  loss  and  molten  slag,  which  are 
impractical  in  a producer.  Steam  in  dissociating  takes  up 
this  excess  sensible  heat,  converts  it  into  latent  heat  and  con- 
veys it  to  the  kiln  without  loss,  which  is  an  essential  factor 
in  ceramic  work  where  the  kilns  are  necessarily  some  dis- 
tance from  the  producer.  At  first  glance  one  would  say  that 
the  reduction  in  volume  of  the  inert  nitrogen,  where  steam  is 
used  in  place  of  air,  would  give  a greater  heat  return  from 
the  fuel,  which  would  be  indicated  by  a higher  ratio  of 

C02 

N 

in  the  stack  gases.  Let  us  assume  that  the  combustion  is 
pure  carbon  and  air.  The  ratio  of 

C02 

N 

in  the  burned  gas  would  be  the  same  as 

O 

N 

from  the  air,  namely, 

21 

79 

by  volume.  This  would  be  the  ratio,  whether  the  combustion 
was  complete  in  the  producer  or  only  partial  in  the  producer 
and  final  in  the  kiln.  Suppose  we  have  pure  water  gas  in 
which  there  is  no  nitrogen.  The  reaction  producing  the  water 
gas  will  be: 

C + H20  = CO  + 2H 

To  burn  the  resulting  gases  we  have: 

CO  -f  2H  + 20  = C02  + H20 

Thus  it  is  seen  that  two  volumes  of  oxygen  are  required 
in  the  secondary  (kiln)  combustion.  The  reactions  without 
the  water  gas  are: 

(1)  C + O = CO 

(2)  CO  + 0 = C02 

Two  volumes  of  oxygen  are  required  in  either  case  and 
with  it  will  be  corresponding  volumes  of  nitrogen.  As  we 


88 


BURNING  CLAY  WARES. 


increase  the  steam,  we  decrease  the  nitrogen  in  the  primary 
(producer)  operation,  but  correspondingly  increase  it  in  the 
secondary  (kiln)  operation.  The  ratio  of  carbon-dioxide  to 
nitrogen  in  the  stack  gases  is  the  measure  of  efficiency  of  the 
combustion,  and  it  is  constant  for  any  fuel,  regardless  of  the 
variations  in  the  producer  operations.  It  varies,  of  course, 
with  excess  or  insufficient  air  in  the  secondary  combustion, 
and  herein  is  its  value  as  a measure  of  efficiency.  It  is  also 
affected  by  any  free  air  passing  through  the  producer,  but 
this  is  a small  matter. 

The  temperature  of  the  gas  leaving  a producer  in  the 
ceramic  industries  varies  from  700°  F.  to  1,500°  F.  The  higher 
the  temperature  in  the  distillation  zone  which  governs  the 
temperature  of  the  exit  gas,  the  better  the  distillation  of  the 
hydro-carbon  products.  On  the  other  hand,  it  increases  the 
radiation  loss  from  producer  and  flues  leading  to  the  kilns. 
The  temperature  is  limited  by  the  character  of  the  coal,  but 
in  many  instances  higher  temperatures  could  be  used  if  it 
were  desirable  to  do  so.  One  authority  places  the  most  effi- 
cient temperature  between  800°  F.  and  900°  F.  In  another 
very  successful  operation  the  temperature  of  the  gas  at  the 
kiln  mouth  was  over  1,100°  F. 

There  is  another  consideration  which  makes  the  exit  tem- 
perature important  and  which  governs  or  should  govern  the 
degree. 

Tar  comes  from  the  distillate  products  and  is  heavier  and 
greater  in  quantity  at  low  temperatures,  and  to  avoid  this  we 
should  have  a high  temperature. 

Soot  comes  from  the  cracking  of  carbon-monoxide  and  it 
is  a maximum  around  900°,  and.  practically  eliminated  at 
1,800°,  which  again  points  to  a desirable  high  temperature, 
but  soot  also  comes  from  the  cracking  of  the  distillate  gases, 
and  from  these  gases  it  is  in  excess  at  high  temperatures  and 
more  voluminous  in  many  instances  than  the  soot  from  the 
carbon-monoxide. 

We  endeavor  to  regulate  the  temperature  to  get  minimum 
tar  and  minimum  soot,  which  for  the  average  bituminous  coal 
is  around  1,000°  F.,  although  at  this  temperature  we  get  the 
soot  from  the  carbon-monoxide,  which  we  must  stand.  The 
temperature  which  gives  a minimum  tar  and  a minimum  soot 
is  called  the  neutral  temperature  and  must  be  determined  for 
each  coal  by  actual  test. 

Nearly  every  one  emphasizes  the  importance  of  the  sen- 


BURNING  CLAY  WARES. 


89 


sible  heat  in  the  gas  in  securing  maximum  kiln  temperatures. 
This  is  beyond  question,  if  it  be  a choice  between  losing  it  by 
radiation  or  utilizing  it  in  the  kiln,  but  if  it  is  a choice  between 
conveying  it  to  the  kiln  as  sensible  heat  or  as  latent  heat,  the 
latter  has  the  advantage,  because  there  is  no  radiation  loss 
in  latent  heat.  Every  producer  gas  user  is  familiar  with  the 
tar  and  soot  trouble,  the  clogging  of  the  flues  and  interrup- 
tion of  the  kiln  operation  in  frequent  burning  out  to  clear  the 
flues,  and  the  annual  job  of  going  through  the  flues  with  chisel 
and  hammer  to  cut  out  the  hard  accumulation  of  residual  car- 
bon, and  each  would  welcome  an  operation  which  would  suc- 
cessfully overcome  these  troubles.  The  process  of  washing 
the  gas  necessary  for  power  gas  loses  the  sensible  heat  as 
well  as  the  value  of  the  tar  products,  and  is  not  to  be  consid- 
ered in  ceramic  work.  Nor  is  it  practical  to  use  the  double 
producer — one  using  bitumnous  coal  and  the  other  coke,  tak- 
ing the  gases  from  the  former  through  the  latter.  As  the 
by-product  industries  develop,  by-product  coke  may  become 
available  in  competition  with  raw  coal,  but  the  time  is  not  yet 
come.  The  process  must  be  one  which  will  take  the  gases 
from  the  producer  coal  and  clean  them,  and  put  back  into  the 
producer  the  sensible  heat  in  pre-heated  air  and  steam.  This 
would  enable  us  to  increase  the  steam,  and  by  means  of  it 
convert  the  additional  heat  into  latent  heat  to  be  conveyed  to 
the  kilns  without  loss. 

This  problem  of  double  operation  in  a single  producer  has 
not  been  solved,  although  several  producers  have  been  de- 
signed which  have  met  with  some  success  and  merit  recogni- 
tion. The  principle  of  the  most  promising  type  is  the  intro- 
duction of  a fire  clay  cylinder  or  retort  in  the  upper  part  of 
the  producer  through  which  the  coal  is  fed.  The  hot  gas 
escaping  from  the  producer  surrounds  this  cylinder  and  heats 
it  and  the  coal  within,  thus  causing  distillation  of  the  volatile 
gases.  The  distillate  is  drawn  off  from  the  top  of  the  cylinder 
and  piped  to  the  bottom  of  the  producer  and  put  back  into 
the  producer  with  the  steam  and  air.  The  producer  is  a com- 
bination of  a gas  retort  and  a coke  producer.  Theoretically, 
it  looks  good,  but  the  practical  man  can  see  several  difficul- 
ties in  its  operation. 

The  operation  of  a producer  to  the  unitiated  seems  a 
very  simple  process — dump  the  coal  in  from  the  top,  remove 
the  ash  from  the  bottom  and  lead  the  gas  to  the  kiln.  It  is, 
however,  not  so  simple.  The  coal  must  be  fed  to  keep  prac- 


90 


BURNING  CLAY  WARES. 


Figure  13.  Figure  14.  Figure  15. 


BURNING  CLAY  WARES. 


91 


tically  constant  the  maximum  efficiency  depth  of  fuel.  This 
means  frequent  feeding  if  the  operation  is  periodic.  If  the 
coal  is  dropped  through  a circular  opening  it  would  pile  up 
in  the  center  of  the  producer  and  the  blast  would  work  to 
the  sides.  See  sketch  Fig.  13.  Such  feeding  would  involve 
frequent  poking  to  keep  the  fuel  bed  reasonably  level. 

A bellhopper  would  give  the  condition  shown  in  sketch 
Fig.  14,  or  in  sketch  Fig.  15,  if  the  hopper  were  large.  The 
need  of  a level  fuel  bed  has  been  met  by  several  spreading 
devices.  The  Bildt  feeder  distributes  the  coal  spirally  from 
center  to  circumference,  and  as  the  feeder  revolves  the  entire 
surface  is  covered.  In  the  Chapman  mechanically  operated 
producer  the  producer  shell  revolves,  but  the  top  and  base 
are  stationary.  The  feed  hopper  is  on  one  side  and  its  diam- 
eter approximates  the  semi-diameter  of  the  producer.  It 
extends  into  the  producer  to  the  top  level  of  the  fuel  bed. 
As  the  fuel  bed  settles,  coal  from  hopper  fills  the  space 
and  the  lower  edge  of  the  hopper  spreads  it  to  a level  sur- 
face. The  entire  surface  of  the  fuel  bed  is  covered  by  the 
revolution  of  the  producer.  The  upper  and  lower  parts  of 
the  producer  are  separate  and  both  revolve  in  the  same  direc- 
tion, but  at  different  speeds.  This  gives  a twisting  motion 
of  the  fuel  bed  upon  itself,  which  keeps  the  fuel  bed  stirred 
up  and  breaks  up  the  ash  clinkers.  A plow  attached  to  the 
lower  edge  of  the  shell  removes  the  ash.  In  some  producers, 
mechanical  stirrers  are  used  to  keep  the  fuel  bed  stirred  even 
down  into  the  ash  pit.  The  point  we  wish  to  bring  out  is  that 
satisfactory  operation  of  the  producer  requires  considerable 
labor  to  keep  the  fuel  bed  in  proper  condition,  to  remove  and 
break  up  the  clinkers,  and  to  remove  the  ash  as  may  be  re- 
quired to  maintain  the  proper  depth  and  to  keep  it  level,  and 
if  the  fuel  is  bad,  high  in  ash  and  sulphur,  it  will  seriously 
interfere  with  the  necessary  continuous  and  uniform  opera- 
tions of  the  producer. 

The  pressure  and  air  volume  are  induced  by  the  aspirating 
effect  of  the  jet  of  steam  which  has  an  initial  pressure  of  sixty 
pounds  to  eighty  pounds.  The  volume  of  air  is  dependent 
upon  pressure  of  the  steam  and  surface  area  of  the  steam  in 
contact  with  the  air.  A small  jet  from  a simple  needle  valve 
will  not  carry  with  it  a sufficient  volume  of  air.  The  develop- 
ment of  the  inspirators  has  been  to  increase  the  surface  area 
of  the  steam  jet.  As  an  illustration  one  type  of  inspirator 
has  a relatively  large  annular  steam  jet  and  air  is  introduced 


92 


BURNING  CLAY  WARES. 


outside  the  jet  as  well  as  through  the  core  of  the  jet,  thus, 
first,  largely  increasing  the  diameter  and  surface  area  of  the 
jet,  and,  second,  doubling  the  efficiency  by  using  both  outer 
and  inner  surfaces.  The  capacity  of  a producer  depends  upon 
the  fuel,  and  the  speed  and  efficiency  of  operation.  We  usu- 
ally estimate  the  capacity  with  an  average  bituminous  coal 
at  ten  pounds  of  coal  per  square  foot  of  grate  area  per  hour, 
though  the  manufacturers  of  some  stationary  producers  will 
guarantee  twelve  pounds  and  even  as  high  as  fifteen,  and 
the  mechanical  producers  can  be  guaranteed  for  twenty-five 
pounds,  and  have  been  operated  up  to  thirty  pounds. 

Grate  bars  are  not  used  in  a number  of  the  successful 
producers  in  this  country,  but  instead  the  ash  collects  on  a 
solid  base  or  in  a pit  sealed  with  water.  The  air  and  steam 
are  introduced  through  a pipe  in  the  ash  pit  to  the  center  of 
the  base,  then  a riser  with  a mushroom  head  terminating  in 
the  upper  part  of  the  bed  of  ashes.  The  head  has  air  ports  to 
distribute  the  air  and  steam  radially. 

The  assumed  grate  area  is  the  area  of  the  base  of  the  com- 
bustion zone.  A six-foot  producer  has  a grate  area  of  twenty- 
seven  feet;  an  eight-foot  producer,  fifty  feet  area;  a ten-foot 
producer,  seventy-eight  feet  area. 

The  smaller  size  may  be  safely  estimated  to  have  a ca- 
pacity of  270  pounds  of  coal  per  hour,  or  3.2  tons  per  day 
(24  hours),  and  the  maximum  capacity  will  be  400  pounds 
per  hour,  or  4.8  tons  per  day.  The  size  of  a stationary  pro- 
ducer is  limited  by  the  ability  of  an  operator  to  keep  the  fuel 
bed  in  proper  condition  by  poking,  and  nine  feet  diameter  is 
the  present  limit  for  hand  stoking,  but  unless  the  fuel  is  ex- 
cellent in  every  respect  a smaller  size  is  preferable.  The 
mechanical  producers  are  practical  in  larger  sizes,  but  we  do 
not  know  of  any  exceeding  ten  feet  in  internal  diameter. 

It  is  not  our  purpose  to  present  a historical  review  of  the 
development  of  the  gas  producer,  nor  to  describe  the  numerous 
producers  on  the  market. 

Such  data  is  available  in  Wyer’s  “Producer  Gas  and  Gas 
Producers,”  in  Nagel’s  “Producer  Gas  Fired  Furnaces,”  in 
Damour’s  “Industrial  Furnaces,”  in  Dowson  & Larter’s  “Pro- 
ducer Gas,”  and  in  technical  journals  and  trade  pamphlets. 
To  take  up  this  interesting  subject  would  only  be  a compila- 
tion and  duplication  from  these  various  publications,  and  of 
greater  interest  to  those  designing  and  building  producers 
than  to  clayworkers  operating  them. 


BURNING  CLAY  WARES. 


93 


The  producer  illustrated  in  Fig.  16  and  Fig.  17,  to  show 
the  general  outline  of  a producer,  and  the  several  producer 
zones,  is  the  Richardson  producer.  It  differs  from  the  usual 
type  of  producer  in  that  it  is  rectangular,  being  five  feet  wide 
and  ten  feet  long,  and  has  two  feed  holes — in  other  words, 
it  is  practically  two  producers  in  one. 


Figure  16. 


It  is  hand-fed,  hand-stoked  and  water  sealed.  The  bosh 
is  backed  up  by  water  cooled  pipes  to  cool  the  ash  and  pre- 
vent excessive  clinkering.  The  air-steam  blast  head  extends 
the  length  of  the  producer,  and  has  escape  ports  in  the  mush- 
room head  from  end  to  end  of  the  producer.  Back  of  the 


i 


BURNING  CLAY  WARES. 


Figure  17. 


BURNING  CLAY  WARES. 


95 


producer  is  built  a large  chamber  in  which  some  of  the  dust, 
soot  and  tar  accompanying  the  gas  will  collect  and  which 
may  be  easily  cleaned.  The  chamber  serves  as  a gas  reser- 
voir, in  which  the  gas  pressure  is  equalized  and  in  conse- 
quence the  puffing  effect  sometimes  noticeable  and  occa- 
sionally troublesome  is  overcome.  Mr.  Richardson  has  in 
view  a simple  and  inexpensive  producer  built  of  bricks  and 
adaptable  to  the  clayworkers’  needs. 

The  losses  in  a producer  are: 

(1)  Heat  in  the  ash. 

(2)  Carbon  in  the  ash. 

(3)  SulpTiur  in  the  ash. 

(41  Generation  of  pressure  steam. 

(5)  Evaporation  of  moisture. 

(6)  Tar  and  soot. 

(7)  Radiation. 

(8)  Dissociation  of  steam. 

(9)  Distillation  and  dissociation  of  hydro  carbons. 

(10)  Sensible  heat  in  the  gas. 

Nos.  1 to  7 are  absolute  losses. 

Nos.  8 to  10  are  recoverable,  in  full  or  in  part,  in  the  kiln. 

The  Ash. 

Since  the  ash  is  cooled  in  water  in  the  producer  hearth, 
its  temperature  will  not  exceed  212°  F.,  and  assuming  62°  F. 
for  the  atmospheric  temperature,  the  loss  in  temperature  is 
150°  F.  In  the  Hocking  Valley  coal  there  was,  including  the 
carbon  and  sulphur,  10.14  per  cent,  ash,  which  at  .16  specific 
heat  would  retain  2.4  B.  t.  u.  per  pound  of  coal  fired — a neg- 
ligible amount. 

Carbon  in  Ash. 

The  chief  loss  in  the  ash  is  the  unburned  carbon,  and  this 
varies  widely.  In  a monograph  issued  by  the  manufacturers 
of  a gas  producer  largely  used  in  the  ceramic  industries, 
the  following  statement  appears : “With  the  better  grades 

of  fuel  probably  a loss  in  the  ash  from  less  than  1 to  5 per 
cent,  of  the  heat  value  of  the  fuel  will  cover  the  range  of 
average  practice.  In  the  poorer  and  finely  divided  coals  it 
may  reach  a much  higher  figure.”  Campbell,  “The  Open 
Hearth  Process,”  Trans.  A.  I.  M.  E.,  Vol.  XXII,  states  that  the 
loss  in  consequence  of  the  carbon  in  the  ash  will  vary  from 
5 to  20  per  cent,  of  the  fuel  fired,  and  in  a specific  case  gives 
the  loss  as  5.6  per  cent.  Bulletin  No.  50,  Eng.  Experiment 


96 


BURNING  CLAY  WARES. 


Station,  University  of  Illinois,  in  tests  of  anthracite  and  coke 
in  a suction  gas  producer,  shows  a variation  from  1.2  to  12 
per  cent,  carbon  in  the  ash  relative  to  the  weight  of  the  fuel. 
Nagel,  “Producer  Gas  Fired  Furnaces,”  says,  “In  the  old 
Siemens  producer  up  to  10  per  cent,  of  the  fuel  was  found 
in  the  ash,  later  producers  showed  a loss  of  5 per  cent.,  while 
in  modern  producers  the  loss  is  said  to  be  reduced  to  1 per 
cent,  or  1.5  per  cent.”  R.  D.  Wood  & Co.  give  the  record 
of  a battery  of  boiler  producers  in  which  the  carbon  in  the 
ash  is  .5  per  cent.  If  we  omit  the  two  highest  amounts,  which 
are  exceptional  and  average  the  remaining  percentages,  we 
get  about  4 per  cent.,  which  would  be  35  per  cent,  of  the  ash, 
but  we  will  figure  on  the  basis  of  25  per  cent.,  the  same  as 
in  the  direct  coal  firing,  although  35  per  cent,  agrees  closely 
with  some  recent  tests  of  producer  work  in  a clayworking 
factory.  There  is  good  practice  and  bad  practice.  The  former 
on  the  best  grades  of  coal  will  show  the  minimum  carbon 
content  in  the  ash,  while  the  latter,  on  an  inferior  coal,  will 
reach  the  maximum  limit.  The  heat  value  of  the  carbon  in 
the  ash  will  be  356  B.  t.  u.  per  pound  of  coal  (Hocking  Valley). 

Sulphur  in  the  Ash. 

There  is  no  data  on  the  sulphur  in  the  ash  and  gas.  As- 
suming as  we  did  in  open  grate  firing  that  19  per  cent,  of  the 
sulphur  in  the  coal  is  retained  by  the  ash,  we  have  from  the 
Hocking  Valley  coal  .0033  pound  of  sulphur  per  pound  of  coal, 
which  in  combustion  would  develop  13  B.  t.  u.  A sulphurous 
coal  is  not  satisfactory  in  producer  work,  in  that  it  tends  to 
produce  a fusible  ash  and  excessive  clinkering. 

Steam  for  Blast. 

As  previously  stated  the  steam  pressure  should  be  between 
60  and  80  pounds,  which  will  give  an  air  blast  in  the  air  pipe 
of  iy2  to  2 pounds  per  square  inch.  It  was  also  estimated 
that  the  quantity  of  steam  should  be  in  excess  of  20  per  cent, 
of  the  weight  of  the  coal.  If  we  assume  20  per  cent,  of  steam, 
every  pound  of  coal  consumed  in  the  producer  will  require 
l/5th  pound  of  steam.  The  evaporation  of  9 pounds  of  water 
per  pound  of  Hocking  Valley  coal  would  approximate  good 
boiler  practice.  The  evaporation  from  a boiler  may  be  roughly 
determined  by  dividing  the  B.  t.  u.  value  of  the  coal  by  970 
and  taking  70  per  cent,  of  the  result.  The  Hocking  Valley 
coal  has  a heat  value  of  12564  B.  t.  u.  and  by  the  above  rule  will 
give  a boiler  efficiency  of  9.1  pounds  of  steam.  The  fuel  con- 


BURNING  CLAY  WARES. 


97 


sumption  for  the  required  steam  will  be  1/9  = X/.20  from 
which  we  find  X = .022  pound  coal  = 276  B.  t.  u.  The  only 
return  from  this  would  be  to  heat  the  air  to  212°  F.  or  what- 
ever the  temperature  would  be  for  the  pressure  in  the  blast 
pipe,  but  it  is  evident  that  the  heat  in  the  steam  will  not 
suffice  to  heat  the  air.  It  can  he  shown  that  there  will  be 
approximately  2.2  pounds  of  nitrogen,  .7  pound  of  oxygen, 
.20  pound  of  steam.  The  steam  at  60  pounds  pressure  will 
have  a temperature  of  307°  F.,  which  reduces  to  212°  F.,  a 
decrease  of  95°  F.  The  air  must  be  heated  from  62°  F.  to 
212°  F.  to  prevent  condensation  of  the  steam.  Roughly  fig- 
ured, the  air  will  require  90  B.  t.  u.  and  the  steam  will  give 
11  B.  t.  u.,  leaving  79  B.  t.  u.  to  be  supplied  by  the  fuel  per 
pound  of  fuel.  Common  practice  uses  more  steam  than  one- 
fifth  pound.  Wyer  gives  one-fifth  pound  as  a requisite  amount, 
while  Nagel  gives  three-eighths  pound  per  35  cubic  feet  of  air 
for  bituminous  coal,  which  would  give  a much  higher  amount 
of  steam  than  one  pound  per  five  pounds  of  coal.  Taylor  gives 
the  same  data  as  Wyer,  but  adds  that  25  per  cent,  more  steam 
may  be  used,  and  that  the  steam  may  be  figured  as  one- 
third  to  two-fifths  of  the  coal  gasified,  which  is  double  our 
estimate,  but  such  a high  volume  of  steam  will  not  give  a 
theoretical  gas  corresponding  with  a standard  gas.  Likely 
some  of  the  steam  is  taken  up  by  the  ash,  and  undoubtedly 
some  is  not  dissociated  and  is  present  in  the  gas  as  water 
vapor. 

Evaporation  of  Moisture. 

The  moisture  in  the  coal  is  evaporated  in  the  distillation 
zone,  and  the  evaporation  may  be  assumed  to  occur  at  212° 
F.  We  determined  .1532  pound  of  moisture  per  pound  of  coal 
and  the  heat  requirement  will  be  149  B.  t.  u.  per  pound  of 
coal. 

Tar  and  Soot. 

The  tar  and  soot  are  variable  and  uncertain  items.  The 
average  of  14  coals  tested  in  St.  Louis  gave  .072  pound  of  tar 
per  pound  of  coal,  figured  on  the  assumption  that  the  tar  had 
a specific  gravity  of  1.2.  H.  H.  Campbell,  previously  cited, 
found  by  actual  test  2.5  grams  in  170  litres  of  the  gas  at  150° 
to  200°  C.  This  figures  .05  pound  per  pound  of  coal  on  the 
assumption  of  55  cubic  feet  of  gas  per  pound  of  coal.  In 
addition  to  this  Campbell  reported  44,000  pounds  of  hard  tar 
in  the  flues,  from  8,000  tons  of  coal.  This  reduces  to  .003 
pound  per  pound  of  coal.  As  the  gas  tested  by  Campbell  was 


98 


BURNING  CLAY  WARES. 


in  use  in  manufacturing  operations,  and  as  the  analysis  of 
the  gas  closely  approximates  a standard  producer  gas,  we 
will  use  his  data.  By  combustion  tests  the  volatile  hydro- 
carbons consisted  of  92.6  per  cent,  carbon  and  7.4  per  cent, 
hydrogen.  We  have,  therefore,  .0463  pound  of  carbon  and 
.0037  pound  of  hydrogen  available  for  combustion  if  the  gas 
is  used  hot,  which  would  not  appear  in  an  analysis  of  pro- 
ducer gas.  This  item  is  usually  overlooked  in  estimating  the 
thermal  value  of  a gas.  The  caked  tar  in  the  flues,  which 
amounts  to  .003  pound  of  carbon  per  pound  of  coal,  is  a dead 
loss.  Since  the  volatile  tar  is  over  90  per  cent,  carbon,  we 
will  not  be  much  in  error  if  we  assume  the  hard  tar  to  be 
carbon,  and  also  assuming  that  the  soot  is  in  equal  amount. 
This  would  give  .006  pound  carbon  to  be  deducted  from  the 
coal  analysis.  The  analysis  of  the  Hocking  Valley  coal  in 
pounds  is  again  given  and  is  as  follows: 


Original  Corrected 


H 

0543 

. . .0506 

C 

6903 

. . .6135 

N 

0126 

. . .0126 

O 

1362 

. . .1362 

S 

0330 

. . .0297 

Ash 

0736 

. . .1014 

Tar 

. . .0560 

The  corrections  in  the  third  columns  are  as  follows: 

Carbon  in  ash 0245 

Carbon  in  V.  tar .0463 

Carbon  in  tar  and  soot 0060 

.0768 

Sulphur  in  ash 0033 

Hydrogen  in  tar 0037 

As  in  the  coal  firing,  we  combine  the  oxygen  with  the  hy- 
drogen to  form  water  vapor,  and  this  requires  .017  hydrogen, 
leaving  .0336  hydrogen.  In  the  distillation  of  illuminating  gas 
we  get  hydrogen  and  methane  in  approximately  the  propor- 
tion of  45  H.  to  CH4,  but  since  this  is  distilled  at  a higher 
temperature,  we  will  not  be  far  wrong  if  we  assume  a 50-50 
distillation.  The  weights  of  hydrogen  in  a 50-50  volume  will 
be  1 hydrogen  as  hydrogen  and  2 hydrogen  in  methane,  and 
from  this  we  get  .0112  hydrogen  as  hydrogen  and  .0224  hydro- 
gen in  the  methane,  and  the  latter  requires  .0672  carbon  and 
develops  .0896  methane.  We  have,  therefore,  .6135 — .0672  = 
.5463  carbon  available  for  carbon-oxides.  In  a standard  pro- 
ducer gas  the  CO  and  C02  are  on  an  average  of  25  per  cent. 


BURNING  CLAY  WARES. 


99 


CO  and  5 per  cent.  C02  by  volume,  and  since  equal  volumes 
of  the  two  gases  contain  the  same  weight  of  carbon,  we  may 
distribute  the  available  carbon  in  proportion  to  the  relative 
volumes  in  the  standard  gas. 

.5463  X 5 

= .4553  C to  CO 

6 

.5463 

= .0910  C to  C02 

6 

.4553  x 

= — = Oxygen  for  CO 

12  16 

x = .6071  Oxygen 
.4553  + .6071  = 1.0624  CO 
.0910  x 

= — = Oxygen  for  C02 

12  32 

x = .2427  Oxygen 
.0910  + .2427  = .3337  Co2 

We  had  .0297  sulphur,  which  will  require  .0297  oxygen, 
producing  .0594  S02.  It  is  possible  chat  all  of  the  sulphur  is 
first  converted  to  H2S,  but  the  affinity  of  sulphur  for  oxygen 
and  the  pungent  odor  of  the  gas  justifies  our  assumption  that 
S02  is  formed  in  the  producer. 

Collecting  the  oxygen,  we  have: 

.6071  for  CO 
.2427  for  C02 
.0297  for  S02 

.8795 

This  oxygen  is  derived  from  the  steam  and  air  We  had 
.20  pound  of  steam  per  pound  of  coal  and  from  this  we  get 
.20  x 

oxygen  in  the  ratio  of  — = — in  which  x = .1778,  and  the 
18  16 

hydrogen  will  be  .0222.  .8795 — .1778  = .7017  oxygen  to  be 

supplied  by  the  air.  The  nitrogen  from  the  air  will  be  in  the 
.7017  x 

proportion  of = x — 2.2843  nitrogen. 

23.5  76.5 

We  have  the  following  results: 


100 


BURNING  CLAY  WARES. 


Water  vapor 1532 

CH4  0896 

S02  0594 

‘ CO  1.0624 

C02  3337 

H 0334 


N 2.2843 

.0126  2.2969 

In  addition  to  this  we  have  in  the  tar  available  for  combus- 
tion .0463  carbon,  .0037  hydrogen. 

The  weights  and  volumes  of  gas  at  atmospheric  pressure 
(29.9  Bar.)  and  62°  F.  are  as  follows: 


Lbs.  per  cu.  ft.  Cu;.  ft.  per  lb. 

Air  

08073 

12.388 

Oxygen  

08921 

11.209 

Hydrogen  

178.931 

Nitrogen  

07831 

12.770 

Carbon-Monoxide  .... 

07807 

12.810 

Carbon-Dioxide  

12267 

8.152 

Methane  

04464 

22.429 

Ethylene  

07809 

12.805 

Water  Vapor  

05020 

19.922 

Sulphur-Dioxide  

17862 

5.600 

Converting  the  gas 

into  volume  we  get: 

Cu.  ft. 

Per  cent. 

Per  cent. 

Water  Vapor  

3.05 

5.3 

CH4  

2.01 

3.6 

3.7 

so2  

33 

.6 

.6 

CO  

13.61 

23.8 

24.7 

co2  

2.72 

4.8 

4.9 

H 

5.98 

10.5 

10.9 

N 

29.33 

51.4 

54.8 

O 

.4 

57.03 

100.0 

100.0 

In  the  second  column  we  have  reduced  the  gas  as  given  to 
per  cent,  volume.  The  values  in  the  third  column  are  for  com- 
parison with  gas  analysis.  Almost  invariably  producer  gas 
has  from  .2  to  .5  per  cent,  of  oxygen,  with  its  accompanying 
nitrogen.  Let  us  assume  .4  per  cent,  oxygen,  which  would  be 
accompanied  by  1.5  per  cent,  nitrogen.  It  is  evident  that,  the 
gas  reduced  to  62°  F.  could  not  contain  5.5  per  cent,  moisture. 
1.69  cubic  feet  of  water  vapor  in  100  cubic  feet  or  1.69  per  cent 
would  be  complete  saturation,  but  gas  analyses  do  not  reckon 
the  water  vapor,  and  it  is  estimated.  These  are  the  correc* 
tions  made  in  the  third  column. 


BURNING  CLAY  WARES. 


101 


Calorific  Value. 

The  calorific  value  of  the  gas  is  as  follows: 


CH4  = .0896  X 26315  = 2358  B.t.u. 
CO  = 1.0624  X 4359  = 4631  B.t.u. 
H = .0334  X 62028  = 2072  B.t.u. 
C in  tar  = .0463  X 14544  = 673  B.t.u. 
H in  tar  = .0037  X 62028  = 230  B.t.u. 


Without  the  tar  we  have.  ..9061  B.t.u. 
With  the  tar  we  have 9964  B.t.u. 


The  first  gives  159  B.  t.  u.  per  cubic  foot  of  gas  and  the 
second  175  B.  t.  u.  The  calorific  value  of  the  coal  as  given  by 
Somermeier  is  12564.  The  value  of  the  gas,  if  used  cold,  in 
terms  of  the  coal,  is  72.8  per  cent,  and  79.3  per  cent,  if  the  gas 
is  used  hot  to  recover  the  volatile  tar.  In  using  the  gas  hot 
we  must  include  the  sensible  heat  of  the  gas.  If  the  gas 
reaches  the  kiln  at  a temperature  of  632  deg.  F.  we  can  de- 
termine the  sensible  heat  from  the  weights  of  the  air  and 
their  thermal  capacities  as  dtermined  in  the  discussion  of 


coal. 

Water  vapor  — .1532  X 256  = 39 
CH2  — .0896  X 442  = 40 
S02  — .0594  X 95  = 6 

CO  — 1.0824  X 149  = 158 
C02  — .3337  X 136  = 45 
H — .0334  X 2083  = 70 
N — 2.2989  X 149  = 342 
Tar  as  CH4  — .0500  X 442  = 22 


722 

This  gives  the  hot  gas  a value  of  10686  B.t.u.,  which  would 
be  85  per  cent,  of  the  value  of  the  coal.  It  is  necessary  to  make 
one  correction  in  each  of  the  three  heat  values.  It  was  noted 
that  276  B.t.u.  were  required  to  generate  steam,  and  it  is 
likely  that  the  average  steam  consumption  will  exceed  this 
assumed  amount  without  giving  any  return  in  heat  value.  We 
then  have  12564+276=12840  B.t.u.  for  every  pound  of  coal 
fired  in  the  producer  and  the  per  cent,  values  will  be  as  fol- 


lows: 

Cold  gas  70. 6 per  cent. 

Hot  gas  and  sensible  heat 83. 2 per  cent. 


We  took  the  thermal  capacity  of  tar  the  same  as  methane 
for  which  we  have  no  authority  but  since  the  total  capacity 
of  the  tar  on  the  basis  of  the  high  calorific  value  of  methane 
is  but  slightly  more  than  one-tenth  of  one  per  cent,  of  the  total 
value  of  the  gas,  we  may  be  widely  in  error  in  this  item  with- 


102 


BURNING  CLAY  WARES. 


out  appreciable  effect  in  the  result.  It  appears  that  gas  In 
its  combustion  must  overcome  a producer  loss  of  71  per  cent, 
before  it  can  show  any  economy,  and  this  loss  is  under  per- 
fect conditions  in  the  producer  reactions.  We  based  our  esti- 
mate on  .2  pounds  of  steam  and  assumed  that  all  of  it  was  dis- 
sociated, which  often  does  not  occur.  Every  tenth  of  a pound 
of  steam  not  dissociated  will  require  165  B.t.u.,  about  IV2 
per  cent,  of  the  value  of  the  gas.  Higher  C02  will  reduce  the 
value  and  conversely  lower  C02  will  increase  the  value.  A 
better  coal  will  give  a higher  value  and  a poorer  coal  a lower 
value.  We  may  gain  or  lose  value  in  the  sensible  heat  depend- 
ing upon  the  distance  of  the  kilns  from  the  producer  and  the 
construction  of  the  flues.  It  is  easy  to  develop  more  fixed  tar 
and  soot  than  we  have  estimated  which  would  increase  the 
loss.  We  reach  the  general  conclusion  that  a safe  estimate 
of  the  average  producer  loss  is  20  per  cent.,  using  the  gas  hot 
as  it  comes  from  the  producer. 

The  chief  advantage  of  gas  is  in  that  it  can  be  burned  with, 
closely  approximating,  the  theoretical  percentage  of  air,  and 
we  get  its  full  value  while  coal  combustion  is  often  accom- 
panied by  a heavy  load  of  excess  air.  Another  advantage 
claimed  for  the  gas  is  that  the  kiln  radiation  loss  is  greatly 
reduced.  Practically  this  is  true  because  kilns  now  in  opera- 
tion are  fixed  in  their  grate  area  and  the  amount  of  coal  that 
can  be  burned  is  limited  while  a much  greater  equivalent  in 
gas  is  possible.  We  could,  of  course,  add  more  and  larger 
furnaces  for  coal  and  thus  reduce  the  percentage  of  radiation 
loss,  but  this  is  beside  the  question. 

As  an  illustration  of  the  advantage  of  firing  a greater  vol- 
ume of  fuel,  assume  that  we  can  attain  and  maintain  a tem- 
perature of  cone  1 in  a kiln,  and  no  more,  and  that  the  radia- 
tion loss  is  40  per  cent,  of  the  fuel.  This  and  the  stack  loss 
require  all  the  coal  we  can  fire.  Suppose  now  we  double  the 
number  of  furnaces,  none  of  the  fuel  fired  in  the  additional 
furnaces  will  be  required  for  radiation  at  cone  1,  and  the  radi- 
ation loss  drops  from  40  to  20  per  cent,  of  the  fuel  fired.  The 
added  fuel  must  bear  its  share  of  stack  losses  but  there  is  a 
balance  of  fuel  available  for  increased  temperature.  As  the 
temperature  advances,  the  added  fuel  must  bear  the  increased 
radiation  and  stack  losses,  and  at  some  higher  temperature  we 
again  reach  a stand  still  point  with  a radiation  loss  of  30  per 
cent,  of  the  total  fuel.  A second  addition  of  fuel  would  drop 
the  radiation  loss  to  20  per  cent.,  but  as  the  temperature  again 


BURNING  CLAY  WARES. 


103 


advanced  the  radiation  loss  would  also  advance  to  25  per  cent. 
In  the  first  instance  40  per  cent,  of  each  pound  of  coal  was 
required  for  radiation  ; in  the  second  at  cone  1,  only  20  per  cent, 
would  be  required ; in  the  third  at  cone  1 the  loss  would 
reduce  to  13  per  cent,  and,  although  we  have  in  the  third 
instance  increased  the  total  stack  and  radiation  losses  in 
consequence  of  the  increased  temperature,  yet  the  percentage 
of  fuel  required  for  the  radiation  loss  at  the  increased  tem- 
perature is  15  per  cent,  less  than  in  the  first  instance.  The 
per  cent,  of  fuel  represented  by  the  stack  losses  is  a factor 
of  temperature  and  for  any  given  temperature  is  constant 
regardless  of  the  quantity  of  fuel  consumed.  Producer  gas 
is  readily  increased  in  its  combustion  volume  and  the  advo- 
cates of  its  use  may  justly  claim  this  advantage — certainly  in 
any  periodic  kiln  now  in  use,  and  the  more  inadequate  the 
kiln  for  coal  firing  the  greater  the  advantage  in  gas  firing. 
The  advantage  of  better  control  and  theoretically  perfect 
combustion  are  decidedly  with  producer  gas  in  competition 
with  coal  in  periodic  kilns. 

Kiln  Temperatures  from  Producer  Gas. 

Kiln  temperatures  are  dependent  upon  the  calorific  value 
of  the  fuel,  the  radiation  and  stack  losses,  and  the  perfect- 
ness of  combustion  of  the  gases.  In  combustion  the  hydro- 
carbons are  burned  to  water  vapor  and  carbon-dioxide,  the 
hydrogen  to  water  vapor  and  the  carbon-monoxide  to  dioxide. 

We  have  in  the  gas: 


ch4  

0896 

H 

0334 

CO  

1.0624 

And  in  the  available  tar: 

C 

0463 

H 

0037 

And  oxygen  required  for  the  first  item  will  be  in  the  pro- 
portion: 

.0896  x 

= — = oxygen 

16  64 

x = .3584  oxygen. 

.0334  x 

The  hydrogen  requires  oxygen  in  the  proportion  = — 

2 16 

x = .2672  oxygen. 

1.0624  x 

The  carbon-monoxide  requires  = — x = .6071 


oxygen. 


28 


16 


104 


BURNING  CLAY  WARES. 


These  three  items  include  all  the  combustibles  if  the  gas 
is  burned  cold  and  the  total  oxygen  required  is  1.2327  and  the 
nitrogen  accompanying  will  be  4.012.  The  reaction  for  CH4  is 

CH4  + 04  = C02  + 2H20.  The  weight  of  CH4  and  O is,  4480 

44  30 

and  the  C02  and  2 HaO  will  be  — and  — , respectively,  times 

80  80 

this  weight,  and  we  find: 

C02  = .2245 
H20  = .1837 

The  hydrogen  produces  H20  = .3006. 

The  carbon  monoxide  gives  C02  = 1.6695. 

Summing  up  and  including  the  inert  gases,  we  have: 


Water  vapor 0460 

S02  0594 

C02  2.2277 

N ....; : 6.3089 

Excess  air 1.3120 

HaO  in  air 0138 


Note : We  use  only  the  amount  of  water  vapor  in  the  cold 
gas  instead  of  the  amount  which  the  coal  would  produce  and 
as  given. 


We  must  now  assume  temperatures  and  determine  the 
thermal  capacities  of  these  gases,  usin? 
the  “Table  of  Thermal  Capacities  of  Gases’ 
sented. 

The  results  are  as  follows: 


HoO 

so2 

CO, 

N 


Excess  of  air  25%  = 752 

Moisture  in  air  = ... . 


using  the  factors  given  in 

of  Gases” 

previously  pre- 

2230  deg. 

2432  deg. 

. 65 

73 

..  32 

37 

..  1577 

1786 

..  4467 

5060 

. 752 

828 

. 19 

22 

6912  B.t.u.  7806  B.t.u. 

The  cold  gas,  less  fuel  for  steam,  has  a value  of  8785.  If 
we  assume  20  per  cent,  radiation  loss  we  have  remaining  7028 
B.  t.  u.  It  is  seen  that  the  temperature  is  slightly  above  2272 
deg.  for  the  cold  gas,  with  25  per  cent,  excess  air  and  20  per 
cent,  radiation  loss.  The  total  of  the  first  four  items  under 
2432  deg.  F.  is  6956,  which  would  be  the  heat  requirement  at 
this  temperature  without  any  excess  air,  and  this  shows  that 
the  temperature  will  be  above  2432  deg.  if  we  burn  the  gas 
with  the  theoretical  volume  of  air. 


BURNING  CLAY  WARES. 


105 


The  hot  gas  has  the  following  combustion  products: 


Water  vapor  6708 

S02  0594 

C02  2.8975 

N 6.8082 

25%  excess  air 1.4743 

H20  in  excess  air .0155 


The  hot  gas,  less  fuel  to  generate  steam,  has  a value  of 
10410  B.  t.  u.  and  20  per  cent,  radiation  loss  will  leave  8328 
B.  t.  u.  We  can  now  determine  the  heat  requirement  for  the 
combustion  gases : 

2432  deg.  2632  deg.  2832  deg.  3032  deg. 


Water  vapor  1063  1186  1315  1449 

SOo  37  42  47  52 

C02  1923  2158  2402  2661 

N 4398  4807  5222  5651 


Total  7421  8193  8986  9813 

Air  930  1017  1104 

Moisture  25  27  30 


Grand  total  . . . .8376  B.t.u.  9237B.t.u.  10120  B.t.u. 

We  note  from  the  first  column  that  the  combustion  gases, 
including  25  per  cent,  excess  air,  require  8376  B.  t.  u.,  and  we 
had  available  8328  B.  t.  u.,  allowing  20  per  cent,  radiation 
loss.  The  possible  temperature  is  therefore  slightly  under 
2432  degrees.  In  the  direct  coal  firing,  allowing  50  per  cent, 
air  excess  and  20  per  cent,  radiation  loss,  we  had  a tempera- 
ture of  2389  degrees.  Just  how  much  less  excess  of  air  would 
be  required  with  the  coal  to  make  the  gas  and  coal  equal 
could  easily  be  determined,  but  the  two  results  given  are 
sufficiently  close  for  comparison.  In  the  second  column  of 
the  gas  combustion  results  it  will  be  noted  that  the  tempera- 
ture will  be  above  2632  degrees  if  we  can  eliminate  the  excess 
air,  but  the  temperature  will  not  reach  2832  degres,  as  shown 
in  the  third  column.  If  we  can  eliminate  both  the  excess  air 
and  radiation  loss  or  reduce  them,  the  temperature  will  be 
above  3032  degrees,  as  shown  in  the  fourth  column.  The 
value  of  gas  compared  with  coal  depends  entirely  upon  how 
much  nearer  we  can  approach  theoretically  perfect  combus- 
tion, and  how  much  we  can  reduce  the  relative  radiation  loss 
by  burning  a greater  volume  in  a given  time. 


106 


BURNING  CLAY  WARES. 


CHAPTER  VI. 

STACKS. 

THE  proper  size  of  a kiln  stack  is  very  uncertain  and  the 
general  practice  is  a rule  of  thumb  determination  but 
without  any  specific  rule  of  thumb. 

A presentation  of  this  problem  with  the  data  available 
will  not  enable  us  to  show  what  is  the  proper  size  of  a kiln, 
stack,  but  it  may  encourage  clayworkers  to  secure  additional 
data  for  calculations  which  will  lead  to  better  stack  designs. 

The  problem  has  been  pretty  well  solved  for  steam  boilers 
and  the  results  presented  in  tables  from  which  we  may  select 
a suitable  stack  for  any  power,  but  this  problem  is  a much 
simpler  one,  in  that  it  deals  with  comparatively  constant 
temperature  and  constant  resistance,  while  in  a klin  stack 
these  factors  are  variable. 

We  are  burning  in  boiler  furnaces  a fixed  amount  of  fuel 
per  hour,  or  per  foot  of  grate  area,  or  per  horsepower,  and 
developing  a fixed  amount  of  gas  per  unit  of  time.  The  tem- 
perature of  the  gas  entering  the  stack  is  practically  constant 
and  is  usually  figured  at  the  maximum  efficiency  temperature, 
550  to  600  degrees  Fahrenheit.  We  have  a thin  metallic  shell 
which  is  a good  conductor  of  heat,  and  within  this  shell  there 
is  circulating  water  with  its  relatively  high  specific  heat  and 
constant  temperature.  The  resistance  in  any  given  boiler  is 
fixed  and  it  is  this  resistance  which  the  stack  power  must 
overcome  and  which  determines  the  required  height  of  stack. 

Since  the  conditions  are  practically  fixed,  the  engineer 
has  only  to  consider  different  grades  and  characters  of  fuels, 
different  climatic  conditions,  and  the  position  of  the  stack 
relative  to  the  boiler,  but  these  introduce  enough  uncertainty 
to  require  serious  consideration  and  modification  of  the 
formulas. 

In  a kiln  operation  the  quantity  of  fuel  for  hollow  ware  is 
increasing  at  a variable  rate  from  start  to  finish  of  the  burn- 


BURNING  CLAY  WARES. 


107 


ing,  and  for  bricks  the  quantity  of  fuel  increases  up  to  the 
soaking  heat  period  during  which  it  is  fairly  constant  to  the 
end  of  the  burning  except  frequently  a slight  falling  off 
toward  the  end.  The  ware  has  a low  specific  heat  and  is  a 
poor  conductor  of  heat.  Moreover,  the  heat  must  be  con- 
ducted from  the  surface  to  the  center  of  the  ware  and  the 
conductivity  is  variable.  The  ware  must  be  brought  to  the 
proper  temperature  in  all  parts  of  the  kiln.  The  escaping 
gases  are  doubly  variable,  increasing  in  volume  with  the 
quantity  of  fuel  and  with  the  temperature.  The  rate  of  fuel 
combustion  is  greater  in  burning  hollow  ware  than  in  burn- 
ing bricks,  but  this  is  a small  difference  compared  with  the 
fuel  requirement  for  different  wares,  the  burning  tempera- 
tures of  which  vary  from  cone  010  to  cone  26.  We  are  careful 
in  increasing  the  furnace  power  for  such  variations,  but  not 
equally  careful  in  proportioning  the  stack. 

Finally  the  resistance  which  the  stack  must  balance  is 
widely  variable.  It  increases  with  the  complexity  of  the  kiln 
bottom,  with  the  distance  of  the  stack  from  the  kiln  and  with 
the  closeness  of  the  set  ware.  A kiln  set  with  bricks  will 
offer  more  resistance  than  one  set  with  sewer  pipe  or  other 
hollow  ware.  Kilns  are  frequently  built  with  stacks  in  the 
kiln  wall  projecting  above  the  heel  of  the  crown  and  the 
gases  in  such  stacks  not  only  have  a maximum  initial  tem- 
perature, but  this  temperature  is  maintained  by  heat  con- 
ducted from  the  kiln  through  the  thin  wall  separating  the 
stack.  The  draft  intensity  of  such  a stack  will  be  very  high. 
Next  will  come  stacks  just  outside  the  kiln  wall,  then  those 
ten  to  twenty  feet  away,  and  finally  those  several  hundred 
feet  away. 

Evidently  the  problem  of  a proper  kiln  stack  is  not  a 
simple  one.  In  fact,  a solution  of  it  is  not  possible,  but  we 
should  at  least  proportion  the  stack  for  conditions  which  are 
known  or  which  can  be  determined,  and  thus  leave  less  to 
guess  work. 

This  is  the  purpose  of  this  presentation. 

Intensity  of  Draft. 

The  intensity  of  draft  of  a stack  is  the  power,  or  suction, 
or  pull,  or  static  pressure,  as  one  may  prefer  to  call  it 

Part  of  the  intensity  is  required  in  the  stack  to  move  the 
weight  of  the  gases  and  overcome  friction  of  the  gases  against 
the  stack  walls,  and  deducting  these  from  the  total  intensity 


108 


BURNING  CLAY  WARES. 


we  get  the  intensity  which  is  available  to  force  the  gases 
through  the  kiln  passages — in  other  words,  to  overcome  kiln 
resistance. 

The  intensity  is  the  difference  in  weight  of  the  stack 
column  of  hot  gases  and  an  equal  column  of  cold  air.  The 
expansion  of  the  hot  gases  creates  a partial  vacuum  in  the 
stack  and  the  outside  air  presses  in  to  overcome  this  vacuum. 
It  is  an  unbalanced  condition — the  light  weight  hot  gases  on 
one  arm  and  the  heavier  cold  air  on  the  other.  If  we  had  a 
stack  alone  the  total  intensity  would  be  used  up  by  the  stack 
resistance.  The  velocity  of  the  gases  in  the  stack  would 
increase  until  the  power  required  to  move  the  weight  of  the 
gases  and  overcome  the  friction  would  equal  the  total  power 
of  the  stack,  which  in  this  case  would  be  the  available  power. 
If  we  connect  the  stack  with  a kiln,  the  velocity  of  the  gases 
in  the  stack  will  decrease  and  make  available  the  necessary 
stack  power  to  overcome  kiln  resistance;  otherwise  there 
would  be  no  draft. 

To  illustrate,  suppose  we  have  a stack  intensity  of  40 
feet  of  air  (stack  intensity  is  indicated  in  feet  of  cold  air, 
in  ounces  pressure,  or  in  inches  of  water),  and  that  the  stack 
requirement  for  any  given  condition  is  five  feet,  leaving  35 
feet  for  the  kiln. 

Let  us  assume  that  ten  feet  of  this  is  required  to  force 
the  air  through  the  furnaces,  fifteen  feet  to  move  the  gases 
down  through  the  ware,  five  feet  to  force  them  through  the 
kiln  floor,  and  five  feet  to  carry  them  through  the  under  floor 
flues  to  the  stack  base.  This  brings  the  gases  to  the  stack 
without  any  force  behind  them  except  that  required  for  the 
stack  and  the  stack  picks  them  up  with  the  force  reserved 
for  it  and  carries  them  to  the  top  of  the  stack.  The  cycle 
is  complete  and  there  is  a perfect  balance.  If  we  should 
open  up  the  grates,  or  in  any  way  reduce  the  resistance  in 
the  kiln,  the  gases  would  arrive  at  the  base  of  the  stack 
with  some  available  force  behind  them,  and  this  force,  added 
to  the  force  which  the  stack  sequesters  for  itself,  would  tend 
to  increase  the  velocity  of  the  gases  up  the  stack  until  this 
excess  force  is  used  up  partly  by  increased  stack  resistance 
and  partly  by  increased  kiln  resistance  in  consequence  of 
greater  velocity  through  the  kiln. 

If  the  kiln  becomes  choked  we  must  rob  the  stack  of  some 
of  its  essential  power  to  overcome  the  increased  kiln  resist- 
ance, which  lessens  the  velocity  in  the  stack,  consequently 


BURNING  CLAY  WARES. 


109 


through  the  kiln,  thus  reducing  the  kiln  resistance.  The 
power  taken  from  the  stack  is  available  to  overcome  the  net 
kiln  resistance. 

Suppose  we  have  a hole,  or  crack,  through  the  kiln  at  the 
base  of  the  kiln  above  the  kiln  floor?  We  have  at  the  en- 
trance to  this  hole  the  same  pressure  as  we  have  under  the 
grate  bars,  while  the  work  ahead  of  it  to  the  base  of  the  stack 
is  only  ten,  whereas  the  work  ahead  of  the  air  entering  the 
grates  is  thirty-five.  If  the  hole  is  large  enough  all  of  the 
air  will  short-cut  through  it  to  the  base  of  the  stack  and  the 
velocity  will  adjust  itself  to  the  total  stack  power,  eliminating 
the  upper  part  of  the  kiln  entirely.  (Note:  In  some  yards 
the  draft  is  controlled  by  a large  opening  in  the  main  draft 
flue  near  the  stack  instead  of  by  slide  dampers.  The  opening 
uncovered  cuts  out  the  kiln  entirely,  but  partially  uncovered 
it  merely  checks  the  draft  through  the  kiln.  Such  an  open- 
ing serves  an  excellent  purpose  in  cooling  the  kiln  by  updraft 
back  through  the  kiln.) 

If  the  hole  in  the  base  of  the  kiln  is  merely  a crack,  the 
sides  of  which  offer  considerable  resistance  to  the  passage  Of 
the  air,  then  the  air  will  rush  through  this  crack  until  a 
velocity  which  gives  a resistance  approximating  25  is  at- 
tained, and  the  operation  will  then  be  in  perfect  balance,  part 
of  the  air  going  through  the  crack  and  part  through  the  kiln. 

The  pressure  within  a kiln  system  is  always  less  than  the 
atmospheric  pressure,  and  one  may  ask  why  it  is  that  gases 
will  escape  through  a hole  in  the  crown,  under  less  pressure 
inside  the  crown  than  the  air  pressure  outside  the  crown. 
The  explanation  is  simple. 

The  kiln  itself  acts  as  a stack  whenever  a hole  is  opened 
in  the  crown,  and  its  power  will  move  the  gases  up  through 
the  hole  in  the  crown  unless  the  main  kiln  stack  has  suffi- 
cient power  to  overcome  the  intensity  of  the  kiln  acting  as  a 
stack  in  addition  to  the  kiln  resistance.  It  matters  not 
whether  the  passage  from  the  air  entrance  to  the  hole  in  the 
crown  is  a direct  one  which  may  be  likened  to  a stack,  or 
whether  it  is  through  a series  of  compartments  as  in  a con- 
tinuous kiln,  so  long  as  the  exit  in  the  crown  is  higher  than 
the  entrance  level  of  the  air. 

This  brings  up  the  question  of  the  value  of  a draft  gage, 
which  C.  B.  Harrop  so  ably  discussed  before  the  American 
Ceramic  Society.  Suppose  we  have  an  ordinary  inclined  tube 
draft  gage  in  the  crown  of  a continuous  kiln,  let  us  say  a 


110 


BURNING  CLAY  WARES. 


chamber  just  connected,  and  which  would  be  cold.  Consid- 
ered as  a kiln  stack,  because  the  chamber  is  cold,  the  inten- 
sity will  be  slight  and  is  easily  overcome  by  the  main  kiln 
draft.  The  gage  will,  therefore,  register  a strong  suction. 

As  the  combustion  approaches  this  compartment,  the  tem- 
perature advances,  and  the  local  upward  intensity  in  the 
compartment  increases,  is  less  overcome  by  the  main  draft, 
and  the  suction  shown  in  the  draft  gage  decreases. 

At  the  maximum  temperature,  and  consequently  the  maxi- 
mum upward  tendency  in  the  compartment,  the  draft  suction 
shown  by  the  gage  may  fall  to  zero,  and  indeed  frequently 
passes  beyond  the  zero  mark,  and  the  gage  registers  ah  out- 
ward pressure  instead  of  a suction.  The  main  kiln  draft 
during  this  period  is  constant,  while  the  draft  gage  shows  a 
regular  decrease,  and  one  must  conclude  that  as  an  indicator 
of  draft  conditions,  the  draft  gage  as  ordinarily  used  is  of 
no  value. 

Area  of  Stack. 

After  a brick  burning  kiln  has  reached  a dull  red  heat  on 
top — 900  to  1200  degrees — the  weight  of  coal  fired  per  hour 
becomes  fairly  constant.  If  the  stack  is  outside  the  kiln  and 
receives  no  heat  except  that  from  the  waste  gases,  the  stack 
temperature  will  be,  perhaps,  232  degrees.  We  may  assume 
that  15  per  cent,  of  the  coal  will  be  used  in  bringing  the  kiln 
up  to  this  stage. 

Red  burning  bricks  will  require  from  200  to  600  pounds  of 
average  coal  (12000  B.  t.  u.)  per  ton  of  bricks,  and  we  will 
take  an  average  of  400  pounds,  less  60  pounds,  for  the  first 
heating  up,  which  gives  us  340  pounds  per  ton. 

The  time  required  to  burn  varies  from  five  to  ten  days, 
and  we  will  assume  eight  days  and  allow  three  days  for  the 
preliminary  heating  up.  A 30-foot  round  down  draft  kiln 
will  hold  230  tons  of  6-pound  standard  sized  bricks,  provided 
the  shrinkage  is  not  excessive.  One  may  figure  fourteen 
bricks  per  cubic  foot  of  setting  space,  or  720  pounds  of  ware 
per  square  foot  of  floor  area.  If  a factory  is  in  operation  and 
wishes  to  increase  the  kiln  capacity,  all  of  the  data  necessary 
for  the  calculations  can  be  obtained  from  the  kilns  in  use  and 
any  faults  in  the  kilns  in  use  can  be  corrected  in  the  new 
kilns. 

If  the  plant  is  a new  one,  data  from  neighboring  factories 
may  be  obtainable,  but  this  lacking,  one  must  depend  upon 
tests  of  the  materials  and  practical  experience. 


BURNING  CLAY  WARES. 


Ill 


In  the  above  example,  we  have  340  pounds  of  coal  per 
hour  during  a period  of  five  days,  starting  with  a stack  tem- 
perature of  232  degrees,  to  determine  the  proper  stack 

We  had  from  the  coal  calculation,  including  50  per  cent, 
excess  air,  14.7  pounds  of  gas,  which  at  62  degrees  will  give 
us  179.4  cubic  feet  and  at  32  degrees,  169.2  cubic  feet.  The 
weight  of  an  equal  volume  of  air  at  62  degrees  is  13.6  pounds, 
and  the  density  of  the  gas  is  therefore  1.08. 

The  volumes  of  gas  at  different  temperatures  above  32 
degrees  are  found  from  the  formula:  V = (1  + .002t)  t. 

We  assumed  340  pounds  of  coal  per  ton  of  ware  and  230 
tons  of  ware  require  78200  pounds  of  coal  during  the  five 
days  of  advanced  firing,  or  .181  pound  per  second  and  the 
volume  of  gas  per  second,  at  32  degrees,  becomes  30.6  cubic 
feet. 

The  volumes  and  density  at  various  temperatures  are  as 
follows: 


Temperature. 

(degrees) 

Volume. 

Density. 

Velocity. 

Ft.  per  Second. 

32 

30.6 

1.080  — 

.... 

232 

42.8 

.771 

6.86 

432 

55.1 

.600  — 

8.82 

632 

67.3 

.491  — 

10.78 

832 

70.6 

.415  + 

12.74 

1032 

91.8 

.360  — 

14.70 

1232 

104.0 

.317  + 

16.66 

1432 

116.3 

.284  + 

18.62 

The  velocity  of  gases  in  factory  chimneys  varies  from  10 
to  20  feet  per  second.  On  the  basis  of  20  feet  per  second, 
and  1432  degrees  temperature,  the  area  of  a stack  for  a 30-foot 
kiln  should  be  5.84  square  feet  and  for  15  feet  per  second  at 
1032  degrees,  the  area  is  6.12  square  feet. 

As  a basis  for  average  conditions,  it  is  our  custom  to  make 
the  diameter,  or  side  of  the  square,  of  the  stack  as  many 
inches  as  the  kiln  is  feet  in  diameter.  If  there  are  two  or 
more  stacks  they  are  proportioned  by  the  equalization  tables. 
Rectangular  kilns  are  reduced  to  equivalent  circles  and  the 
diameters  of  these  circles  taken.  A 30-foot  kiln  stack  would 
by  this  rule  have  a diameter  of  30  inches  and  the  area  would 
be  4.91  square  feet  and  a square  stack  would  be  30  inches 
square,  or  6.25  square  feet.  The  fourth  column  in  the  above 
table  gives  the  velocities  in  a 2-foot  6-inch  square  stack  under 
the  assumed  conditions. 

It  is  evident  that  for  high  temperature  ware  where  the 
coal  consumption  will  run  up  to  600  or  more  pounds  per  ton 


112 


BURNING  CLAY  WARES. 


in  the  same  period  of  time  as  we  assumed  in  our  calculation, 
resulting  in  high  stack  temperature,  the  stack  area  should 
be  greater  than  that  given  by  our  rule,  but  on  the  other  hand 
high  temperature  ware,  such  as  silica  bricks,  in  order  to  get 
the  necessary  temperature,  is  burned  with  a deep  fuel  bed 
and  a scant  supply  of  air,  which  reduces  the  volume  of  gas 
per  pound  of  coal,  and  this  must  be  taken  into  consideration. 

The  proper  method  for  getting  the  stack  area  is  to  deter- 
mine the  quantity  of  fuel  per  second,  convert  this  into  gas 
volume  at  the  required  temperature,  and  proportion  the  stack 
area  for  a gas  velocity  not  exceeding  20  feet  per  second. 

Total  Draft  Intensity. 

The  draft  intensity  is  the  difference  in  weight  between  the 
stack  column  of  hot  gas  and  an  equal  column  of  cold  air.  We 
will  assume  a stack  height  of  60  feet.  The  gas  weighs  .087 
pound  per  cubic  foot  and  the  air  partly  saturated  may  be 
taken  as  .0806  pound  per  cubic  foot,  both  at  32  degrees. 

The  respective  weights  of  60-foot  columns  of  air  at  32 
degrees  and  gas  at  632  degrees  will  be: 

60  X .0806  = 4.836  pounds  of  air. 

60  X .087 

— = 2.373  pounds  of  gas. 

1 _j_  .002  X 600 

Intensity  = Diff.  = 2.463  pounds  = .2737  ounce  per  sq.  inch. 

Another  method  which  enables  us  to  get  the  result  in  feet 
of  cold  air  direct  and  thus  eliminates  the  decimal  errors  in 
converting  ounces  into  feet  of  air  is  as  follows: 

(Wt.  of  cu.  ft.  of  air  — Wt.  of  cu.  ft.  of  gas  corrected 
for  temp.) 

I = H — 

Wt.  of  cu.  ft.  of  air. 

The  results  for  32  degrees  air  temperature  are  shown  in 
the  following  table: 


Total  Intensity  of  a 60  Foot  Stack. 


Head  in  Feet 

Head  in  Inches 

Head  in 

Temperature. 

of  Air. 

Water. 

Ounces. 

232 

13.7 

.21 

.12 

432 

24.0 

.38 

.22 

632 

30.5 

.47 

.27 

832 

35.1 

.53 

.31 

1032 

38.4 

.59 

.34 

1232 

40.9 

.62 

.36 

1432 

42.9 

.66 

.38 

A cu.  ft.  of  water  at  32  degrees  weighs  62.418  pounds. 

at  39.1  degrees  weighs  62.425  pounds, 
at  62  degrees  weighs  62.355  pounds. 


BURNING  CLAY  WARES. 


113 


If  we  assume  62.4  pounds  for  the  water  and  .0806  pound 
for  air,  a cubic  foot  of  water  equals  774  cubic  feet  of  air,  and 
an  inch  of  water  will  weigh  0.578  ounce.  These  factors  will 
enable  us  to  convert  feet  of  air  into  inches  of  water  or  ounces, 
and  vice  versa. 

If  we  multiply  the  feet  of  air  by  12  to  get  it  in  inches  and 
divide  the  product  by  774,  we  will  get  the  pressure  in  inches, 
and  if  we  multiply  this  result  by  .578,  we  get  the  result  in 
ounces  pressure.  For  example,  we  found  30.5  feet  of  air  for 
632  degrees. 

30.5  X 12 

= .473  inch  of  water. 

774 

.473  X .578  = .2734  ounces. 

By  direct  calculation,  using  weights,  we  had  .2737  ounce. 

The  method  of  calculation  for  any  temperature,  reduced  to 
simple  formulas,  would  be  the  following: 
w W 

I = .111  H ( ) = pressure  in  ounces. 

1 + .002t  1 + .002T 
w W 

I ==  12.4  H ( ) = pressure  in  feet  of  air. 

1 -f  .002t  1 + .002T 
w W 

I = .192  H ( ) = pressure  in  inches. 

1 + .002 1 1 + 002T 
I = intensity  or  static  pressure. 

H =height  of  stack. 

W = weight  of  cu.  ft.  of  air  at  32°. 

W = weight  of  cu.  ft.  of  gas  at  32°. 
t = Temperature  of  air  above  32°. 

T = Temperature  of  gas  above  32°. 

Velocity  Head. 

The  intensities  or  heads  above  determined  are  totals,  and 
Dart  of  this  pressure  is  required  to  lift  the  weight  of  gasea 
and  maintain  the  velocity  of  their  passage  through  the  stack 
flue,  and  another  part  is  required  to  overcome  the  stack  re- 
sistance. The  velocity  head  is  determined  by  the  formula: 

V2d 

h = 

2g(l  + -002T) 

h = velocity  head  in  feet  of  air. 

V = velocity  of  gas  up  the  stack, 
d = density  of  gas  at  32  degrees, 
g = acceleration  of  gravity,  32.14  feet  per  second 
T = stack  temperature. 


114 


BURNING  CLAY  WARES. 


The  height  of  the  stack  does  not  enter  into  this  proDiem 
since  we  wish  to  determine  the  force  required  to  move  a 
certain  weight  at  a fixed  velocity.  For  632  degree  stack  tem- 
perature h = .888. 


Friction  Head. 

The  friction  requirement  is  determined  from  the  formula: 
H 

F = Velocity  head  X — X K. 

S 

H = Height  of  stack. 

S = diameter,  or  side  if  square,  of  stack. 

K = factor  which  varies  between  .05  for  a smooth  stack 
and  .12  for  a rough  stack  as  determined  by  Grashof.  Richards 
uses  an  average  value  of  .08  for  K,  but  as  kiln  stacks  are 
usually  very  rough,  we  will  use  .10.  The  friction  head  for  632 
60 

degrees  will  be  .888  X X .10  = 2.131  feet  of  air  for  an 

2.5 

assumed  stack  60  feet  high  and  2 feet  6 inches  square. 

The  results  for  the  conditions  we  have  been  considering 
are  as  follows: 


Velocity,  Friction  and  Available  Heads. 


Temp. 

Vel.  Head  in 
Feet  of  Air. 

Friction  Head 
in  Feet  of  Air. 

Available  Head 
in  Feet  of  Air. 

232 

.564 

1.354 

11.8 

432 

.726 

1.741 

21.5 

632 

.888 

2.131 

27.5 

832 

1.048 

2.515 

31.5 

1032 

•1210 

2.904 

34.3 

1232 

1.368 

3.283 

36.3 

1432 

1.531 

3.674 

37.7 

Maximum  Efficiency  Temperature. 

The  engineering  manuals  give  from  550  to  600  degrees 
Fahrenheit  as  the  maximum  efficiency  temperature,  or  as  gen- 
erally stated  the  maximum  weight  of  gases  can  be  moved  at 
this  temperature. 

The  variable  is  in  the  available  intensity  and  not  in  the 
stack  requirement.  If  we  multiply  the  stack  resistance 
(velocity  head  plus  friction  head)  by  the  density  which  gives 
the  relative  weight  of  gas  moved,  the  result  will  be  a con- 
stant for  all  temperatures.  If  we  multiply  the  total  bead  or 
available  head  by  the  density  the  maximum  will  be  found 
at  632  degrees,  probably  less  than  this  if  we  make  the  cal- 


BURNING  CLAY  WARES. 


115 


culation  for  degrees  intermediate  between  432  and  632  de- 
grees. Our  calculations  are  based  on  a fixed  amount  of  coal 
per  second,  but  if  we  allow  the  stack  to  regulate  the  amount 
of  coal  burned,  the  maximum  will  be  at  632  degrees  stack 
temperature,  provided  the  conditions  in  the  kiln  are  constant, 
as  would  be  practically  the  case  in  a boiler  equipment. 

Maximum  efficiency  temperature  means  that  a stack  will 
move  a maximum  weight  of  gases  at  632  degrees  against  a 
fixed  resistance  equal  to  the  available  head  at  that  tempera- 
ture, which  in  our  problem  is  27.5  feet  of  air,  but  it  does 
not  mean  a stronger  draft  at  632  degrees,  because  if  the 
stack  temperature  is  1432  degrees  and  the  draft  is  satisfac- 
tory, we  could  not  increase  the  draft  by  chilling  the  stack 
temperature  to  632  degrees.  On  the  contrary,  this  would  im- 
mediately cause  the  furnaces  to  smoke  for  the  reason  that 
the  1432  degree  temperature  is  overcoming  a resistance  of 
37.7  feet  of  air,  and  if  we  checked  the  stack  temperature  to  a 
point  where  it  could  only  overcome  a resistance  of  27.5  feet 
of  air,  there  would  be  a resistance  of  10.2  feet  which  the 
stack  could  not  overcome  and  the  movement  of  the  gases 
through  the  kiln  would  have  to  be  reduced  until  this  excess 
resistance  was  eliminated. 

Chilling  the  stack  gases  would  have  the  same  effect  as 
excessively  lowering  a damper.  The  rate  of  combustion  would 
fall  off  to  a degree  less  than  required  to  maintain  the  kiln 
temperature  and  the  kiln  would  cool.  As  the  kiln  cooled  the 
rate  of  combustion  would  pick  up  again,  but  not  sufficient  to 
overcome  the  rate  of  cooling  until  the  temperature  in  the  kiln 
is  lowered  to  that  degree  which  it  originally  had  when  the 
stack  gases  were  normally  632  degrees.  At  this  temperature 
we  will  have  the  maximum  rate  of  combustion  and  we  will  be 
moving  the  maximum  weight  of  gases  through  the  stack.  If 
we  were  evaporating  water  this  stationary  632  degree  condi- 
tion would  have  the  maximum  efficiency,  in  other  words,  we 
could  evaporate  a greater  quantity  of  water  in  a given  time. 

Our  periodic  kiln  burning  problem  is  an  entirely  different 
one.  At  some  stage  in  the  burning  process  we  have  the  632 
degree  condition,  but  as  the  temperature  of  the  kiln  mounts 
higher  and  higher  and  the  kiln  resistance  increases  in  con- 
sequence, the  temperature  of  the  stack  gases  automatically 
increases  and  necessarily  must  do  so  to  overcome  the  in- 
creased kiln  resistance  and  approximately  maintain  the  rate 
of  combustion.  The  rate  of  combustion  falls  off  somewhat 


116 


BURNING  CLAY  WARES. 


above  632  degree  stack  temperature,  but  it  is  of  no  conse- 
quence if  we  have  ample  furnace  power.  Our  aim  is  to  get 
a maximum  kiln  temperature,  and  to  do  this  we  must  in- 
crease the  stack  power  as  the  kiln  temperature  increases,  and 
this  is  accomplished  by  increased  stack  temperature. 

Maximum  efficiency  temperature  is  readily  shown,  but  it 
is  not  applicable  to  periodic  kiln  burning.  It  does  apply  to 
continuous  kiln  operation,  because  in  such  we  have  constant 
conditions,  fully  so  in  the  tunnel  type  of  kiln  and  approxi- 
mately in  the  chambered  type. 

The  following  table  in  the  first  six  columns  collects  the 
data  which  we  have  already  given. 

In  the  seventh  column  we  multiply  the  total  head  by  the 
density  which  gives  the  weight  of  gas  moved  for  each  tem- 
perature. It  will  be  noted  that  the  maximum  is  at  632 
degrees. 

In  the  eighth  column  we  multiply  the  available  head,  that 
is,  the  total  head  minus  the  stack  requirement,  by  the  density 
and  get  the  maximum  weight  again  at  632 


Stack  Heads  and  Weights  of  Gas  Moved.. 
Stack  2 Feet  6 Inches  Square,  60  Feet  High. 


<X> 

U 

0 

g 

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0) 

M 

03 

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0 

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KA  fl 

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m u 

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H 

03 

<V 

M 

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as 

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•O  . 

03 

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8 

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•g 

3 

03  Q 

QJ 

e 

CD 

> 

EH 

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§ X 

^ x 

232 

.564 

1.354 

1.918 

13.7 

11.8 

13.56 

9.10 

432 

.726 

1.742 

2.467 

24.0 

21.5 

14.40 

12.90 

632 

.888 

2.131 

3.019 

30.5 

27.5 

14.98 

13.50 

832 

1.048 

2.515 

3.553 

35.1 

31.5 

14.57 

13.07 

1032 

1.210 

2.904 

4.114 

38.4 

34.3 

13.82 

12.34 

1232 

1.368 

3.283 

4.651 

40.9 

36.3 

12.97 

11.51 

1432 

1.531 

3.674 

5.205 

42.9 

37.7 

12.18 

10.71 

Stack  Furnaces. 

Some  kiln  builders  use  a small  furnace  directly  connected 
with  the  stack  to  increase  the  stack  intensity.  This  it  does, 
but  at  the  same  time  it  introduces  an  additional  volume  of 
gas  and  thus  uses  some  of  the  power  which  it  creates.  To 
whatever  extent  air  enters  through  the  stack  furnace,  to  the 
same  extent  is  the  draft  through  the  kiln  lessened,  tempera- 


BURNING  CLAY  WARES. 


in 


ture  not  considered.  The  size  of  the  furnace  is  not  impor- 
tant, but  the  opening  into  the  stack  should  be  of  such  small 
size  that  the  resistance  equals  or  exceeds  the  kiln  resistance, 
depending,  of  course,  on  the  volume  of  hot  gas  from  the 
furnace  required  to  heat  the  volume  of  gas  from  the  kiln  to 
the  desired  temperature. 

There  is  a legitimate  use  for  such  a furnace  in  the  early 
stages  of  the  burning  when  the  stack  power  is  very  low,  and 
yet  at  the  same  time  we  need  a strong  draft  to  sweep  the 
moisture  out  of  the  kiln  during  the  early  stages  of  water- 
smoking, and  during  oxidation  when  we  need  a large  excess 
of  air  for  the  oxidation. 

Two  or  more  kilns  on  one  stack  with  a flue  for  each  kiln 
have  the  advantage  that  the  stack  is  kept  hot  and  in  starting 
a kiln  we  have  the  value  of  this  temperature,  which  the  kiln 
in  question  could  not  give  during  the  early  stages  of  firing. 

Two  kilns  should  never  be  connected  with  a single  stack 
flue.  When  we  have  one  kiln  connected  it  has  the  full  power 
of  the  stack  controlled,  of  course,  by  a damper.  When  we 
connect  a second  kiln  we  rob  the  first  kiln  of  some  velocity 
of  movement  of  gases  through  the  kiln,  since  we  must  divide 
the  volume  of  gases  moved  proportionately  between  the  two 
kilns,  and  besides  the  second  kiln  being  cold,  lowers  the 
available  head  of  the  stack.  Such  conditions  are  undesir- 
able if  for  no  other  reason  than  the  irregularity  of  the  opera- 
tion. 


Height  of  Stack  for  Periodic  Kiln. 

Data  is  lacking  to  develop  any  definite  rule  to  be  used 
in  the  determination  of  the  heights  of  kiln  stacks. 

The  stack  temperature  and  the  kiln  resistance  are  the 
two  items  necessary  in  order  to  establish  such  a rule.  These 
can  be  determined  in  any  operating  plant,  but  the  data  may 
apply  only  measurably  to  another  plant  in  which  the  setting 
is  different,  offering  greater  or  less  resistance;  in  which  a 
different  type  of  furnace  is  to  be  used;  in  which  the  quantity 
of  fuel  burned  per  hour  is  greater  or  less;  in  which  there  is 
a more  or  less  complicated  kiln  bottom. 

If  we  had  data  from  a number  of  kiln  operations  under 
different  conditions,  one  approximating  the  desired  condition 
could  be  selected  and  with  some  corrections,  perhaps,  could 
be  used,  but  such  data  is  entirely  lacking. 


118 


BURNING  CLAY  WARES. 


We  can  get  some  idea  of  the  approximate  height  from  the 
following  method  of  calculation.  The  formula  for  total  stack 
intensity  is 

Wt.  of  Air  — Wt.  of  gas 

I = H ( ). 

Wt.  of  Air. 

V2d 

The  formula  for  velocity  heads  is  h = 

2g. 

hHK 

and  for  friction  head,  F = . 

S 

We  wish  to  determine  the  height,  H. 

The  problem  should  be  solved  for  some  average  atmos- 
pheric temperature,  let  us  assume  62  degrees,  at  which  a 
cubic  foot  of  air  will  weigh  .076  pound.  The  gas  at  32  de- 
grees weighs  .087  pound. 

Prof.  Gale,  in  a test  of  a boiler,  found  the  total  static 
pressure  to  be  0.48  inch  water  pressure  distributed  as  fol- 


lows: 

Entrance  Velocity  0.5% 

Great  Resistance  36.6% 

Boiler  Tubes,  etc 49.5% 

Discharge  Velocity  3.4% 

Flue  to  Stack 2.4% 

Stack  Friction  7.6% 


100.0% 

Stacks  based  on  a kiln  resistance  equal  to  boiler  resistance 
would  make  it  possible  to  burn  the  coal  at  a rate  of  15  pounds 
of  coal  per  hour  per  square  foot  of  grate  surface,  which  is 
good  boiler  practice.  The  rate  of  combustion  in  kiln  work  is 
scarcely  one-half  the  rate  in  boiler  work,  and  if  we  consider 
the  stack  solely  from  the  standpoint  of  the  rate  of  combus- 
tion, the  stack  heights  could  be  materially  reduced. 

It  must  be  remembered,  however,  that  the  average  tem- 
perature of  the  gases  in  a kiln  is  much  higher  than  in  a 
boiler  equipment  and  the  frictional  resistance  is  greater  in 
consequence,  leaving  less  available  stack  intensity  for  the 
furnace  resistance. 

A test  of  a gas  burning  kiln  stack  showed  .2-inch  stack 
intensity.  There  was  no  furnace  resistance,  but,  to  the  con- 
trary, the  gas  was  introduced  under  pressure,  and  this  pres- 
sure was  available  to  assist  in  overcoming  the  kiln  resistance. 
The  sum  of  gas  pressure  and  the  stack  intensity  exceeded 


BURNING  CLAY  WARES.  119 

• 

.35-inch  of  water  pressure,  and  the  test  indicates  that  a coal 
burning  kiln  set  with  bricks  will  offer  a resistance  greater 
than  .4-inch  of  water  pressure. 

It  also  must  be  borne  in  mind  that  during  the  water  smok- 
ing and  oxidation  stages,  which  are  prior  to  the  maximum 
intensity  stage  of  the  stack,  there  is  a large  volume  of  water 
vapor,  and  often  sulphur-dioxide  and  carbon-dioxide  from  the 
ware,  which  must  be  considered.  Water  vapor  being  lighter 
than  air  increases  the  stack  intensity,  but  the  other  two  are 
approximately  double  the  weight  of  air. 

Finally,  in  the  best  practice  it  is  customary  to  partially 
close  the  damper  during  the  finishing  stages  of  the  burning, 
thus  introducing  a resistance  which  must  be  offset  by  less 
kiln  resistance,  or,  in  other  words,  checking  the  kiln  draft 
The  stack  must  be  designed  for  the  worst  condition  of  the 
burning  operation,  and  controlled  for  the  more  favorable  con- 
ditions. 

We  will  not  be  far  astray  if  we  adopt  the  boiler  data  for 
our  periodic  kilns.  The  two  items  of  grate  resistance  and 
tube  resistance  total  86  per  cent.,  which  in  round  numbers 
require  a stack  intensity  of  .4-inch  of  water,  plus  the  stack 
requirement.  This  kiln  requirement  in  feet  of  air  will  be 
.4  X 774 

= 25.8  feet 

12 

The  total  intensity  required  will  be  25.8  -f  velocity  head  + 
friction  head,  which  will  equal 

Wt.  of  Air  — Wt.  of  Gas. 

H ( ). 

W t.  of  Air. 

If  we  assume  a stack  temperature  of  1,432  degrees,  the 
second  member  of  the  above  equation  becomes : 

.076  — .087  X .284 

X H,  which  reduces  to  .675H. 

.076 

The  velocity  in  the  stack,  as  previously  determined,  is 
18.62  feet  per  second. 

The  velocity  head  becomes 

18.642  X .284  X 1.06 

= 1.6 

64.3 

Note — We  wish  to  determine  the  velocity  head  in  feet  of 
air  at  62  degrees,  and  therefore  the  densities  as  previously 
given  must  be  multiplied  by  1 + .002  t = 1.06,  because  .284 


120 


BURNING  CLAY  WARES. 


is  the  density  relative  to  air  at  32  degrees,  whereas  relative 
air  at  62  degrees  it  will  be  .284  X 1.06  = .301. 

We  have  already  determined  the  velocity  heads  relative 
to  32  degrees  and  need  only  to  correct  these  values  for  62 
degrees,  namely,  1.531  X 1.06  = 1.6  feet  velocity  head  for  1,432 
degrees.  For  a stack  2 feet  6 inches  in  diameter,  or  square, 
the  friction  head  is 

H 

1.6  X X .1  = .06  H. 

2.5 

We  now  have : 


Total  stack  intensity  = .675  H 

Kiln  and  furnace  resistance  =25.8 

Velocity  head  = 1.6 

Friction  head  = .06  H 

We  wish  to  find  the  value  of  H.  The  last  three  items 

equal  the  first,  which  gives  us  the  equation,  .675  H = 25.8  + 
1.6  + .06  H,  from  which  we  find  H = 45  feet. 


The  distance  of  the  stack  from  the  kiln  introduces  flue 
resistance  equivalent  to  the  stack  friction  per  foot,  which  we 
have  found  to  be  .06. 

We  must  introduce  this  into  the  equation,  and  let  us  as- 
sume flue  lengths  of  15  feet  and  300  feet. 

For  the  first  equation  we  have: 

.675  H = 25.8  + 1.6  + .06  H + .06  X 15. 

H = 46  feet. 

For  the  second  equation: 


.675  H = 25.8  + 1.6  + .06  H + .06  X 300. 

H = 74  feet. 

In  kiln  wall  stacks  we  have  even  higher  temperatures  than 
1,432  degrees.  Since  the  stack  is  separated  from  the  interior 
of  the  kiln  by  a 4%-inch  wall,  or  at  most  9 inches,  and  since 
combustion  gases  are  burned  in  the  stack  at  least  a part  of 
each  firing  period,  the  stack  temperature  will  approximate 
the  kiln  temperature.  Wall  stacks  then  may  be  correspond- 
ingly low. 

On  the  other  hand,  the  greater  the  distance  of  the  stack 
from  the  kiln,  the  cooler  the  gases  and  the  higher  the  stack 
requirement. 

The  following  table  gives  the  heights  for  different  stack 
temperatures,  in  the  first  column  without  flue  resistance,  in 
the  second  including  15  feet  of  flue,  and  in  the  third  including 
300  feet  of  flue. 


BURNING  CLAY  WARES.  121 


Temp. 

Height 

Height 

Height 

in  stack 

without  flue 

15  ft.  flue 

300  ft.  flue 

432 

94 

95 

125 

632 

66 

67 

93 

832 

56 

57 

80 

1032 

50 

51 

77 

1232 

47 

48 

76 

1432 

45 

46 

74 

Stacks  within  15  feet  of  the  kiln  will  usually  have  tem- 
peratures between  832  and  1032  degrees,  and  stacks  300  or 
more  feet  away  will  have  the  temperature  reduced  to  632 
degrees  or  less. 

In  working  out  the  proper  height  of  stack  the  engineer 
must  take  into  consideration  and  allow  for  a number  of  fac- 
tors which  cannot  be  included  in  a typical  calculation. 

The  above  calculations  are  based  on  the  consumption  of 
.18  pound  of  coal  per  second.  An  increased  coal  consumption 
will  have  no  effect  on  the  grate  resistance,  since  it  would 
involve  increased  grate  area,  but  it  does  involve  iu creased 
kiln  resistance  because  we  would  thereby  increase  the  volume 
of  gases  passing  through  the  kiln,  thus  increasing  the  velocity 
which  is  a factor  of  the  friction.  We  provide  a larger  stack 
area  for  increased  volumes  of  gas,  but  we  cannot,  or  do  not, 
provide  a larger  free  area  in  the  kiln,  and  therefore  the  stack 
should  be  higher  to  overcome  the  increased  kiln  resistance. 
On  the  other  hand,  we  cannot  increase  the  velocity  of  the 
gases  through  the  kiln  without  in  some  degree  increasing  the 
stack  temperature,  and  this  will  lower  the  height  of  stack 
required. 

We  assumed  a kiln  resistance  based  on  a limited  boiler 
test,  and  this  may  not  be  a reasonable  assumption.  Our 
justification  is  that  the  results  approximate  good  practice  in 
clay  ware  burning. 

Our  assumption,  further,  is  for  a down  draft  kiln,,  set  with 
brick.  Hollow  ware  would  have  less  resistance  and  would 
not  require  as  high  a stack. 

We  have  determined  the  weight  of  the  kiln  gas  at  32  de- 
grees to  be  .087  pound  per  cubic  foot,  and  noted  that  the 
weight  varies  between  .085  and  .087,  and  if  we  use  the  lignter 
weight  we  increase  the  stack  intensity  and  reduce  the  height 
of  the  stack. 

Our  calculations  are  based  on  coal  burning  furnaces,  which 
offer  resistance  and  require  additional  stack  height  in  con* 


122 


BURNING  CLAY  WARES. 


sequence.  Where  the  fuel  is  natural  gas,  producer  gas  or  oil, 
it  is  introduced  into  the  furnace  box  under  pressure,  thus  not 
only  eliminating  furnace  resistance,  but  actually  developing 
a pressure  available  to  overcome  kiln  resistance,  lessening 
the  work  to  be  done  by  the  stack  and  reducing  the  required 
stack  height  correspondingly. 

The  stack  construction  is  important.  The  intensity  of  the 
stack  depends  upon  the  average  temperature  of  the  gases 
in  the  stack,  and  the  temperature  is  lowered  by  increased 
radiation  and  leaks. 

The  situation  also  must  be  considered,  the  height  and 
position  of  adjacent  buildings  or  hills  and  the  direction  of 
prevailing  winds. 

The  height  of  kiln  wall  stacks  varies  from  6 to  8 feet  above 
the  kiln  crown,  or  a total  of  18  to  24  feet.  Stacks  just  out- 
side the  kiln  walls  range  in  height  from  25  to  50  feet  depend- 
ing upon  the  type  of  kiln.  We  have  built  rectangular  kilns 
100  feet  long  with  stacks  at  the  end,  and  the  long  draft  flue 
had  to  be  reckoned  with  in  deciding  the  height  of  the  stack. 
Stacks  15  to  18  feet  away  from  the  kiln  are  built  from  40 
to  60  feet  high,  not  considering  long  draft  flues  inside  kiln, 
as  mentioned  above.  When  the  stacks  are  several  hundred 
feet  away  from  the  kiln — a single  stack  for  a battery  of  kilns 
— the  height  should  be  determined  for  the  most  distant  kiln 
and  will  be  from  80  to  100  feet  high. 

Continuous  Kiln  Stack. 

A modern  chambered  continuous  kiln  is  a series  of  down 
draft  compartments  comparable  with  a periodic  kiln.  Each 
compartment  has  bags,  checkered  floor,  under  floor  flues,  and 
the  setting  is  the  same  as  in  down  draft  kilns. 

The  furnace  resistance  of  a coal  fired  periodic  kiln  is 
eliminated  in  the  continuous  kiln. 

From  the  limited  data  available,  it  would  not  be  safe  to 
figure  on  less  than  0.3  inch  water  pressure  for  each  compart- 
ment of  a continuous  kiln  during  its  maximum  temperature 
period. 

We  will  assume  capacity  of  30,000  brick  per  day,  and  a 
fuel  (coal)  consumption  of  400  pounds  per  thousand  brick, 
making  12,000  pounds  per  day,  or  .139  pounds  per  second.  We 
will  also  assume  100  per  cent,  excess  air. 


BURNING  CLAY  WARES. 


123 


The  Hocking  Valley  coal,  per  pound,  under  such  condi- 
tions, will  give  us  at  32  degrees : 

Dry  Air  114.3  cubic  feet 

C02  20.6  cubic  feet 

N 90.3  cubic  feet 

S02  3 cubic  feet 

Water  Vapor  13.5  cubic  feet 

239.0  cubic  feet 

The  total  weight  of  the  gas  is  19.5  pounds,  and  the  weight 
per  cubic  foot  is  .082  pound,  which  gives  a density  of  1.02. 

The  cubic  feet  of  gas  are  as  follows : 


Per  Pound  Per  Second 

Entering  Air  236  cubic  feet  32.8  cubic  feet 

Combustion  Gas  239  cubic  feet  33.2  cubic  feet 


We  will  assume  the  following  temperature  in  seven  con- 
nected chambers,  viz.,  232,  932,  1632,  2132,  1232,  832,  632,  and 
an  average  stack  temperature  of  432.  The  first  three  com- 
partments is  the  combustion  compartment ; the  last  three  are 
heating  up. 

Correcting  the  air  and  gas  for  temperature,  assuming  that 
the  air  enters  at  62  degrees,  we  determine  the  following  rela- 
tive velocities  per  second : 

1.32,  2.64,  3.96,  4.97,  3.25,  2.48,  2.1. 

The  densities  will  be : 

0.75,  0.32,  0.25,  0.2,  0.3,  0.4,  0.48. 

The  relative  resistance  is  as  the  velocities  squared  times 
the  densities,  and  in  this  way  we  get  the  following  results : 
1.31,  2.58,  3.92,  4.94,  3.17,  2.46,  2.12. 

The  total  relative  resistance  is  the  sum  of  the  above,  and 
is  20.5. 

The  combustion  compartment  has  a relative  resistance  of 
4.94  and  the  actual  resistance  assumed  is  0.3. 

The  total  kiln  resistance  will  be  in  the  proportion  of 
4 94  20.5 

.3  x 

x = 1.245  inch  water  pressure  = 80.3  feet  of  air. 

The  volume  of  gas  per  second  at  32  degrees  is  33.2  cubic 
feet,  which  at  the  stack  temperature  becomes  59.76  cubic  feet. 

If  we  assume  a stack  diameter  of  4 feet,  the  velocity  in 
the  stack  will  be  4.8  feet  per  second. 


124 


BURNING  CLAY  WARES. 


The  total  stack  intensity  will  be : 

.076  — 0.455 

H = .4H 

.076 

The  velocity  head  will  be: 

4.82  X 6 


.215 


64.3 


The  friction  head  is : 


.0215 


H = .0054  H 


We  get  the  equation : 

.4  H = 80.3  + .215  + -0054  H, 

From  which: 

H = 204  feet. 


In  order  to  build  a stable  stack  for  such  heights  the  diam- 
eter must  be  large  which  reduces  the  velocity  and  friction 
heads  to  a negligible  factor,  and  we  may  determine  the  height 
from  the  total  intensity  and  the  required  pressure  which  in 
the  above  example  would  give  201  feet  for  the  height  of  the 
stack. 

We  have  figured  on  seven  compartments,  which  is  a prac- 
tical minimum  often  exceeded,  and  for  a greater  number  of 
compartments  we  must  increase  the  height  of  the  stack. 

The  modern  compartment  continuous  kiln  is  complicated 
compared  with  earlier  types  of  kilns,  and  the  resistance  is 
correspondingly  greater,  which  explains  why  we  have  resorted 
to  induced  draft.  The  earlier  kilns  had  solid  floors,  no  checker 
work,  no  under  floor  flues,  and  the  compartments  were  con- 
nected by  relatively  large  direct  openings.  The  ware  was  set 
with  flues,  longitudinal  in  the  lower  part  of  the  kiln,  and  verti- 
cal feed  holes  at  short  intervals  throughout  the  mass  of  ware. 

Tunnel  kilns  of  the  Hoffman  type  have  solid  floors,  and  no 
division  walls,  and  flues  are  set  in  the  ware.  The  resistance 
in  such  a kiln  would  be  less  although  the  gases  are  pulled 
through  a longer  distance.  The  latter  type  of  kiln  may  be 
likened  to  a long,  tortuous,  exceedingly  rough  flue.  If  we 
figured  on  the  basis  of  equivalent  0.2  resistance  per  compart- 
ment in  a tunnel  kiln,  the  stack  would  be  136  feet  high.  A 
resistance  of  0.25  per  compartment  would  require  a stack 
158  feet  high. 

Stacks  for  the  earlier  types  of  compartment  kilns  and  the 
tunnel  kilns  vary  from  125  feet  to  190  feet  in  height.  In  the 


BURNING  CLAY  WARES. 


125 


earlier  types  of  compartment  kilns  the  stack  was  frequently 
included  within  the  kiln  walls  and  its  base  was  kept  hot  by 
conduction  from  the  adjacent  compartments.  In  both  types 
the  main  draft  flue  was  above  the  ground  in  the  longitudinal 
center  of  the  kiln,  with  the  compartments  or  tunnels  on  either 
side,  and  thus  the  stack  gases  averaged  a higher  temperature 
than  that  entering  the  main  draft  flue,  which  increased  the 
draft  intensity.  In  spite  of  these  aids,  we  frequently  found  it 
necessary  with  stacks  of  minimum  height  to  open  a damper 
into  the  first  compartment  ahead  of  the  combustion  compart- 
ment in  order  to  strengthen  the  draft  by  increasing  the  tem- 
perature of  the  stack  gases. 

The  discussion  brings  up  a point  worthy  of  mention.  In 
modern  kilns  we  by-pass  the  air  for  water  smoking,  the  pur- 
pose of  which  is  to  insure  the  water  smoking  keeping  pace 
with  the  burning,  to  reduce  the  scumming  difficulty  and  to 
overcome  the  swelling  peculiar  to  a continuous  kiln  product. 
The  hot  air  is  taken  from  a cooling  compartment  and  con- 
ducted through  a flue  direct  to  the  compartments  ahead. 

The  air  leaving  the  cooling  compartment  has  an  excessive 
force  and  the  flue  offers  little  resistance.  The  total  resistance 
in  this  circuit  may  be  the  resistance  of  a single  compartment, 
or  two  compartments,  or  three,  depending  upon  whether  they 
are  connected  to  the  main  draft  flue  singly  or  in  series.  The 
temperature  in  these  compartments  is  relatively  low,  and  the 
resistance  less  in  consequence. 

These  low  resistance  compartments  are  connected  with  a 
stack  or  fan  adapted  to  overcome  the  resistance  through  a 
longer  series  of  higher  temperature  compartments. 

The  effect  is  to  weaken  the  drift  intensity,  in  consequence 
of  the  reduced  stack  temperature,  and  because  of  the  increased 
volume,  although  this  is  a small  matter  if  the  stack  area  is 
amply  large. 

The  proper  adjustment  is  obtained  by  introducing  damper 
resistance,  but  this  is  uncertain  at  best. 

The  control  of  two  distinct  and  widely  differing  operations 
with  a single  equipment  emphasizes  the  importance  of  greater 
flexibility,  which  a fan  gives,  and  it  is  but  a step  to  the  use 
of  two  fans,  one  for  each  operation 

We  have  not  attempted  to  go  into  the  details  of  the  prob- 
lem. For  instance,  the  clay  will  contain  5 to  10  per  cent,  of 
combined  water  and  3 per  cent,  or  more  of  hygroscopic  water, 
which  as  vapor  in  the  gas  will  lower  the  density,  and  lessen 


126 


BURNING  CLAY  WARES. 


the  height  of  stack  required.  On  the  other  hand,  carbon  or 
sulphur  in  the  clay  would  increase  the  density  of  the  gas, 
unless  they  were  included  in  the  combustibles  and  provided 
for,  otherwise  they  would  convert  excess  air  into  heavier  car- 
bon and  sulphur  dioxides. 

Construction  of  Stacks. 

The  bearing  weight  of  soil,  clay,  gravel,  etc.,  are  usually 
taken  as  follows: 

One  ton  per  square  foot  for  soft  wet  soil. 

Two  tons  per  square  foot  for  firm,  wet  soil  and  sand. 

Three  tons  per  square  foot  for  firm,  dry  soil,  clay  or  fine 
sand. 

Four  tons  per  square  foot  for  dry,  hard,  coarse  sand,  clay 
or  gravel. 

This  leaves  a wide  margin  for  the  exercise  of  one’s  judg- 
ment, and  the  rule  should  be  to  err  on  the  safe  side. 

Hotop’s  formulas  for  the  depth  and  size  of  stack  founda- 
tions and  foundation  base  plates  are  as  follows: 

Depth  of  foundation,  one-eighth  of  the  height  of  the  stack 
above  ground. 

Breadth  of  foundation,  one-eighth  of  the  total  height  of 
stack. 

Thickness  of  foundation  plate  (below  the  flue  entrance), 
1.6  + .01  H. 

A 60-foot  stack,  by  these  rules,  would  have  a depth  below 
ground  of  7.5  feet.  The  breadth  would  be  8.4  feet,  and  the 
thickness  of  the  base  plate  2.24  feet. 

A better  method  is  to  estimate  the  weight  of  the  stack  and 
determine  the  foundation  dimensions  from  this  data. 

A 60-foot  stack  with  a 4%-inch  lining  and  outside  walls 
13  inches,  9 inches  and  4 y2  inches  thick  in  sections  each  20 
feet  high  and  a foundation  depth  of  7 feet  will  weight  70  tons. 

The  maximum  breadth  of  the  base  for  soft,  wet  soil  will 
be  8.2  feet.  On  the  basis  of  2 tons  per  square  foot,  the  re- 
quired breadth  of  the  foundation  is  5.8  feet,  but  if  the  stack 
has  a 30-inch  flue  the  side  at  the  base  will  be  6 feet  5 inches, 
and  with  proper  set-off  at  the  bottom,  together  with  a proper 
slope  or  steps  in  the  base,  we  will  get  the  maximum  breadth. 
The  angle  of  the  slope  should  not  be  less  than  30  degrees 
from  the  vertical. 


BURNING  CLAY  WARES. 


127 


Concrete  should  not  be  used  in  the  stack  foundation  above 
the  bottom  of  the  kiln  draft  flue,  nor,  indeed,  nearer  than 
12  inches  to  the  bottom  of  the  flue,  on  account  of  the  high 
temperatures  which  develop  in  the  stack.  If  the  ground  is 
soft  and  wet,  it  is  desirable  to  carry  the  foundation  deeper 
in  order  to  use  a monolithic  concrete  base  plate,'  but  as  a rule, 
if  the  ground  is  suitable  for  kilns  which  have  underground 
flues  to  a depth  of  from  4 to  8 feet,  the  stack  foundation  will 
be  on  hard,  dry  soil  or  gravel  and  there  is  little  need  of  a 
massive  base  plate.  The  foundation  above  the  base  plate 
should  be  built  of  hard  burned  brick  laid  in  cement  mortar. 

Every  kiln  stack  should  have  an  independent  fire  brick 
inwall.  We  have  stack  temperatures  exceeding  1,000  degrees, 
and  if  the  wall  is  single  the  expansion  inside  lifts  the  outer 
brick  from  their  bed,  and  once  loosened,  they  are  easily  sep- 
arated laterally,  developing  zig-zag  cracks  from  top  to  bottom. 

The  fire  brick  inwall  need  not  be  over  4 V2  inches  thick 
unless  the  stack  is  very  high,  and  the  brick  should  be  laid  in 
a thin  bed  of  fire  clay  mortar  in  the  lower  part  of  the  stack 
and  adding  some  cement  to  the  clay  mortar  for  the  upper  part. 

The  end  joints  may  be  heavier,  in  fact,  they  should  be,  to 
provide  locally  for  the  expansion  of  each  brick. 

The  top  of  the  inwall  should  be  several  inches  below  the 
top  of  the  outer  wall,  so  that  the  expansion  will  not  lift  any 
protective  cover  placed  on  the  outer  wall. 

The  outer  wall  at  the  base  should  be  set  back  about  6 
inches  from  the  inwall  and  be  carried  up  with  a batter  inside 
and  out  until  the  inside  is  approximately  2 inches  from  the 
inwall,  then  drop  off  one  brick  in  the  thickness  of  the  wall 
without  breaking  the  continuity  of  the  batter  of  the  outside 
face  and  carry  this  wall  to  the  proper  height  to  drop  off  an- 
other brick  in  the  thickness. 

Tne  height  of  each  section  should  be  about  25  feet  and  not 
exceeding  30  feet.  A 50-foot  stack  may  start  with  a 9-inch 
wall,  dropping  off  to  a 4%  inch,  and  similarly  a 60-foot  wall, 
but  the  latter  preferably  should  have  three  sections,  13  inches, 
9 inches,  and  4 y2  inches  thickness  of  wall  respectively.  Stacks 
exceeding  75  feet  in  height  should  have  no  section  less  than 
a 9-inch  wall,  not  considering  the  fire  brick  inwall. 

The  outside  wall  should  be  laid  in  cement  or  lime-cement 
mortar.  An  excellent  lime  mortar  for  kiln  work  is  made  of 
lime  and  ground  furnace  clinkers  instead  of  sand.  The  clink- 
ers are  much  sharper  than  sand  and  have  some  hydraulic 
property. 


128 


BURNING  CLAY  WARES. 


The  top  of  the  outer  wall  should  have  a suitable  cap,  either 
iron  or  reinforced  concrete,  and  this  should  overhang  inside 
to  cover  the  space  between  the  outer  and  inner  wall  and  par- 
tially cover  the  inner  wall.  It  is  customary  to  put  holes  in 
the  base  of  the  outer  wall  for  the  admission  of  air  to  keep 
the  inside  face  of  the  wall  cool. 

At  intervals  of  four  to  six  feet  there  should  be  a brick 
projection  from  the  center  of  the  inner  face  of  the  outer  wall 


to  the  stack  lining  wall,  but  not  bonded  into  the  lining  wall. 
It  is  better  to  make  this  projection  about  three  courses  high 
and  to  chamfer  the  top  and  bottom  edges.  The  purpose  of 
these  projections  is  to  stay  the  fire  brick  lining,  yet  at  the  same 
time  not  to  interfere  with  the  rise  and  fall  of  the  lining  as  it 
expands  and  contracts  under  changes  of  temperature.  Fig. 
No.  18  illustrates  a properly  constructed  stack. 

Induced  Draft. 

The  use  of  fans  in  place  of  stacks  is  now  very  common. 
They  are  used  almost  exclusively  in  continuous  kiln  opera- 


BURNING  CLAY  WARES. 


129 


tions,  and  are  beginning  to  replace  periodic  kiln  stacks.  The 
advantages  of  fan  drafts  are: 

1.  Less  first  cost. 

2.  Flexibility,  in  that  by  speeding  up  the  fan,  we  increase 
the  intensity  of  the  draft  and  the  volume  of  gases  handled. 

3.  Independence  of  atmospheric  conditions. 

In  continuous  kiln  operations  it  is  imperative  that  the 
stacks  be  replaced  by  fans  in  order  to  increase  the  rate  of 
burning. 

There  is  a limit  to  the  height  of  a stack,  if  for  no  other 
reason  than  the  cooling  of  the  gases  to  a temperature  at 
which  the  weight  of  cubic  foot  of  gas  is  equal  to  or  greater 
than  the  weight  of  a cubic  foot  of  outside  air.  To  make  a 
higher  stack  effective  it  would  be  necessary  to  take  the  gases 
from  the  kiln  at  a higher  temperature,  which  would  involve  a 
loss  of  heat. 

The  need  is  not  so  imperative  in  periodic  kilns  because  of 
the  high  stack  temperatures,  but  in  such  kilns,  in  view  of  the 
fact  that  we  need  greater  intensity  than  a stack  will  give 
during  the  earlier  stages  of  the  burning,  the  use  of  a fan  has 
decided  advantages  over  a stack.  The  loss  in  heat  in  a pe- 
riodic kiln  stack  is  a large  percentage  of  the  total  fuel,  and 
this  loss  is  unavoidable  in  a stack,  since  the  heat  is  repuired 
to  create  the  draft.  With  fan  draft  we  may  make  use  of  this 
heat,  and  extended  use  of  fans  for  periodic  kilns  will  lead  to 
the  development  of  uses  for  the  waste  heat  in  the  gases. 

Dryers  have  been  built  in  which  the  combustion  gases  from 
the  kilns  are  taken  through  flues  in  the  dryer  and  the  heat 
applied  to  drying  the  ware.  Here  is  one  direct  use  of  the 
waste  heat,  which  with  fan  draft  is  readily  adaptable,  but 
which  is  not  applicable  to  stack  draft  because  of  the  increased 
resistance  introduced  by  the  dryer  flues  and  the  decreased 
stack  intensity  in  consequence  of  the  lower  temperature  stack 
gases.  Such  an  application  of  waste  heat  to  radiated  heat 
dryers  is  but  a step  in  advance  of  the  present  direct  firing. 

The  collection  of  the  waste  heat  in  the  combustion  gases 
in  one  or  two  kiln  installations  has  been  accomplished  by  an 
economizer,  and  this  idea  is  worthy  of  further  development. 

It  would  be  practical  to  use  the  heat  thus  collected  for 
primary  or  secondary  air  in  the  kiln  furnaces. 

Some  designs  have  been  worked  out  to  generate  steam  for 
power  with  the  waste  gases  under  the  kiln  floor,  which  would 


130 


BURNING  CLAY  WARES. 


not  be  satisfactory  because  of  the  cooling  effect  on  the  floor, 
but  there  can  be  no  objection  to  using  the  heat  for  such  pur- 
pose. Such  conservation  of  waste  heat  is  in  practical  opera- 
tion. 

The  selection  of  the  fan  for  induced  draft  requires  study 
and  thought.  We  must  first  determine  the  volume  of  gas  to 
be  moved,  provide  for  maximum  conditions,  and  make  some 
allowance  for  leakage. 

The  theory  of  fan  performance  is  a little  puzzling  to  a 
layman,  and  each  problem  should  be  put  up  to  a manufac- 
turer of  fans,  but  unfortunately  we  cannot  give  them  satisfac- 
tory data.  All  tables  of  fan  performances  presented  in  fan 
catalogues  are  for  ventilation  and  are  only  measurably  appli- 
cable to  clayworking  conditions. 

A brief  discussion  of  fan  operation  will  not  be  out  of  place. 

Mergue,  Rateau  and  others  discuss  the  problem  from  the 
standpoint  of  effective  area  or  epuivalent  orifice. 

In  Fig.  19,  the  large  square  represents  the  outlet  of  a fan. 
The  shaded  area,  which  is  40  to  50  per  cent,  of  the  total  area, 
is  the  effective  area,  or,  in  other  words,  it 
is  the  area  of  opening  within  which  we 
can  maintain  a constant  static  pressure 
without  changing  the  speed  of  the  fan.  If 
we  assume  a desired  static  pressure  of 
one  inch  of  water  for  any  operation,  we 
cannot  get  this  with  a fully  opened  outlet. 
If  we  close  the  outlet  entirely,  the  fan 
will  develop  a pressure  of  one  inch  within 
the  closed  space.  When  this  pressure  is  attained  the  back- 
ward pressure  through  the  fan  just  balances  the  forward  pres- 
sure by  the  fan.  If  we  open  the  outlet,  increasingly  up  to 
an  outlet  equal  to  the  shaded  area,  the  pressure  within  the 
space  will  remain*  practically  constant,  and  the  effect  is  an 
increase  in  volume  of  air.  If  we  are  designing  an  equipment 
with  several  distributed  outlets  as  a dryer,  for  instance,  the 
outlets  should  have  an  equivalent  area  equal  to  about  40  per 
cent,  of  the  fan  outlet,  and  thus  we  will  get  approximately 
the  volumes  listed  and  the  pressure  throughout  the  main  dis- 
tributing duct  will  approximate  the  estimated  pressure. 

If  we  have  a fan  connected  with  itself  by  an  encircling 
duct,  as  shown  in  Fig.  20,  there  will  be  no  static  pressure  in 
the  duct  if  it  is  equal  to  the  fan  outlet  throughout  the  circuit, 
friction  not  considered.  If  we  introduce  a diaphragm  as  at 


Fig.  19. 


BURNING  CLAY  WARES. 


131 


“A,”  this  when  completely  closed  enables  the  fan  to  develop 
its  maximum  pressure,  say,  one  inch,  between  the  diaphragm 
and  the  fan  and  this  static  pressure  relatively  will  be  main- 
tained up  to  a 40  per  cent,  diaphragm  opening,  above  which 
the  pressure  falls  off  rapidly  to  zero  at  the  full  opening.  If 
we  move  the  diaphragm  to  “B,”  there  is  no  change  in  prin- 
ciple, except  we  say  that  the  fan  is  overcoming  a resistance 


Fig.  20. 


of  one  inch  instead  of  developing  a like  pressure.  Suppose 
now  we  remove  the  air  from  the  duct,  there  being  nothing  to 
move,  no  pressure  can  be  developed.  If  we  introduce  a little 
gas  we  can  develop  a slight  pressure,  increasing  with  the 
density  of  the  gas  until  at  atmospheric  conditions  we  again 
develop  the  maximum  pressure. 

The  point  of  this  is  that  the  pressure  given  in  catalogue 
data  is  based  on  atmospheric  conditions,  and  if  we  wish  to 
handle  a hot,  rarified  gas,  we  must  select  an  equipment  with 


132 


BURNING  CLAY  WARES. 


a higher  listed  pressure  in  order  to  overcome  the  equivalent 
resistance.  It  is  simply  a question  of  force  equals  mass  times 
velocity.  If  we  increase  or  decrease  the  mass  we  increase  or 
decrease  the  force  which  we  measure  as  pressure. 

Effective  areas  assume  that  the  pressure  is  constant  up 
to  40  per  cent,  opening,  but  this  is  not  strictly  true. 

The  per  cent,  pressure  curve  for  a steel  plate  fan  may  be 
represented  by  the  solid  line  curve  shown  in  Fig.  21.  With  no 
outlet  we  get  the  initial  pressure,  which  increases  slightly  with 
an  opening  approaching  20  per  cent.  Then  follows  a pres- 
sure with  slight  change  up  to  50  per  cent,  opening,  beyond 
which  there  is  a rapid  decline  in  pressure.  The  effective  area 


is  this  range  of  slight  change,  and  we  preferably  take  the  up- 
per limit  in  order  to  get  a maximum  volume. 

The  dotted  curve  illustrates  the  behavior  of  a multi-vane 
fan.  First  a slight  drop,  then  a rise,  followed  by  the  effective 
area  between  30  and  60  per  cent,  opening. 

In  working  out  a problem  for  a pressure  system,  we  select 
a fan  with  the  required  volume  and  pressure  capacity  and 
proportion  the  delivery  outlets  to  suit  the  fan. 


BURNING  CLAY  WARES. 


133 


We  cannot  do  this  for  induced  draft,  which  would  mean 
constructing  the  kiln  and  setting  the  ware  to  correspond  with 
the  fan.  We  must  determine  the  kiln  resistance,  the  volume, 
density  and  temperature  of  the  gas.  In  pressure  estimates  we 
figure  on  40  per  cent,  effective  area,  but  since  we  do  not  know 
what  the  area  is  in  induced  work,  we  should  err  on  the  safe 
side  and  base  our  estimates  on  an  area  not  exceeding  30  per 
cent.  The  determined  resistance  must  be  increased  to  cor- 
respond with  the  decrease  in  gas  density  compared  with  air, 
then  select  from  the  fan  tables  the  fan  which  will  deliver  the 
volume  of  gas,  temperature  considered,  and  which  will  de- 
velop a static  pressure  equal  to  the  resistance.  This  pres- 
sure is  a matter  of  speed,  and  the  selection  should  be  an  aver- 
age speed  which  gives  the  desired  flexibility  for  a fan  equip- 
ment. 

In  our  periodic  kiln  stack  problem  we  assumed  a kiln 
resistance  of  .4  inch  of  water.  Let  us  say  that  the  kiln  re- 
sistance, including  flue  to  fans,  turns  in  flue,  etc.,  totals  .5  inch 
resistance  and  that  the  gases  reach  the  fan  with  a tempera- 
ture of  832  degrees.  We  estimated  on  the  basis  of  .181  pounds 
of  coal  per  second,  which  in  round  numbers  would  be  11 
pounds  per  minute  developing  about  1970  cu.  ft.  of  gas.  This 
increases  to  5122  cu.  ft.,  or  allowing  for  leakage,  assume  6000 
cu.  ft.,  at  832  degrees,  and  the  density  becomes  .38.  The  re- 
sistance then  should  be  estimated  1.32  inches  water  pressure. 
The  fan  (steel  plate)  should  have  a 4y2-foot  wheel  speed  350 
r.p.m.,  or  a 5-foot  wheel  speed  300  r.p.m. 

If  we  should  figure  on  four  kilns,  the  fan  should  have  a 
9-foot  wheel  with  a speed  of  190  r.p.m. 

This  gives  an  excellent  starting  point  in  the  selection  of 
the  fan,  and  the  results  from  available  data  and  carefully 
considered  assumptions  will  apply  to  our  clayworking  opera- 
tions. 

In  the  selection  of  a single  fan  for  the  double  operation 
involved  in  continuous  kiln  operation,  we  must  make  wide 
allowance  for  imperfect  operation.  We  can  determine  within 
a reasonable  approximation  the  volume  of  gases  from  the 
burning  operation,  and,  given  the  temperature,  we  can  deter- 
mine the  air  volume  of  the  water-smoking,  but  we  cannot  deter- 
mine the  uncertain  control  of  the  latter.  No  harm  results  if 
we  use  ten  times  the  required  volume  in  the  water-smoking 
except  the  decrease  in  the  profits  through  excessive  waste  of 
heat. 


134 


BURNING  CLAY  WARES. 


It  can  be  shown  that  it  requires  nearly,  perhaps  twice,  as 
much  air  for  the  water-smoking  as  that  required  for  the  burn- 
ing. 

Assume  30,000  bricks  containing  3 per  cent,  moisture  aDd 
2 per  cent,  hygroscopic  water  heated  up  to  an  average  tem- 
perature of  432  degrees  by  air  entering  at  632  degrees  and 
leaving  at  332  degrees. 


180.000  pounds  brickwork  X .2  X 185 6,660,000  B.t.u. 

171.000  pounds  dry  clayware  X .2  X 370 12,654,000  B.t.u. 

5,400  pounds  water  X 150  810,000  B.t.u. 

5,400  pounds  water  to  vapor  X 970 5,238,000  B.t.u. 

5,400  pounds  water  vapor  X .48  X 220 570,240  B.t.u. 

3,600  pounds  water  (hygroscopic)  X 370 1,332,000  B.t.u. 

3,600  pounds  water  X (966  — .7  X 200)....  2,973,600  B.t.u. 


30,237,840  B.t.u. 

Less  3,600  X .49  X 100 • 176,400  B.t.u. 


Total  heat  requirement 30,061,440  B.t.u. 


If  the  air  enters  the  kiln  80  per  cent,  saturated,  the  thermal 
value  from  each  pound  will  be  72.5  B.  t.  u.,  and  to  give  the 
above  heat  requirement  we  must  have  405,537  pounds  of  air 
per  day,  or  4.7  pounds  per  second,  or  58  cubic  feet  per  second. 
The  moisture  from  the  ware  adds  2 cubic  feet,  making  60 
cubic  feet  per  second.  We  found  the  air  for  combustion  to 
be  33  cubic  feet  per  second,  and  to  this  2 cubic  feet  of  com- 
bined water  vapor  must  be  added,  making  35  cubic  feet 

In  one  instance  where  the  theoretical  combined  volume  of 
gases  did  not  exceed  18,000  cubic  feet  per  minute,  the  actual 
volume  coming  through  the  fan  exceeded  50,000  cubic  feet, 
showing  a big  loss  in  heat  and  power. 

In  the  problem  of  stack  height  we  found  a required  pres- 
sure of  1.245  inches  of  water.  If  the  gases  entering  the  fan 
have  a temperature  of  432  degrees,  the  density  will  be  ap- 
proximately 0.56-inch.  We  have  35  cubic  feet  of  gas  per 
second  from  the  combustion  compartments  and  60  cubic  feet 
from  the  water-smoking,  making  a total  of  95  cubic  feet  of 
gas  per  second,  or  5,700  cubic  feet  per  minute.  At  432  de- 
grees this  becomes  9,920  cubic  feet.  We  must  select  a fan 

1.245 

speed  which  will  give  a static  pressure  of = 2.4  inches 

.56 

and  the  volume  .will  be  9,920  cubic  feet  plus  allowance  for 
leakage  and  bad  control. 


BURNING  CLAY  WARES. 


135 


Theoretically,  this  would  require  a steel  plate  fan  with 
a 6-foot  wheel,  running  at  something  over  400  r.  p.  m.  If 
we  allow  three  times  the  estimated  volume  for  leakage  and 
bad  control,  which  was  the  fact  in  the  test  cited,  the  opera- 
tion will  require  a fan  with  a 9-foot  wheel,  running  about 
250  r.  p.  m. 


136 


BURNING  CLAY  WARES. 


CHAPTER  VII. 

FURNACES. 

THE  FURNACE  is  the  most  important  part  of  a kiln 
and  upon  it  largely  depends  the  economy  and  success 
of  the  burning. 

There  are  two  general  types — the  flat  grate  and  the  in- 
clined grate,  or  more  properly,  pit  furnaces — which  include 
nearly  all  the  furnaces  in  common  use  in  clayware  kuns. 

The  common  flat-grate  furnace,  such  as  we  find  in  boiler 
furnaces,  is  too  well  known  to  need  any  description. 

The  selection  of  the  grate  bar  best  adapted  to  the  purpose 
is  an  important  item  in  economical  operations  and  many  plants 
may  net  a comfortable  profit  by  attention  to  this  detail.  The 
ordinary  double  parallel  bar  has  not  good  distribution  of  the 
metal  and  in  consequence  its  life  is  short  and  the  waste  in 
fuel  excessive.  The  herring-bone  bars  with  wide  side  ribs 
are  better,  although  objectionable  in  that  the  proximity  of  the 


ribs  in  adjacent  bars  does  not  give  full  opportunity  for  cool- 
ing, which  is  necessary  to  keep  the  bars  from  sagging  in  the 
center.  A better  type  has  a single  central  deep  rib  with 
eheckerwork  top,  Fig.  22. 

The  rocking  grates,  of  which  there  are  several  on  the 
market,  will  prove  a profitable  investment  over  the  usual 
fixed  type  of  bar.  The  average  clayworker  looks  askance  at 
the  cost  of  an  installation,  but  views  with  complacency  a seri- 
ous fuel  loss  simply  because  the  loss  is  charged  to  burning 
and  does  not  appear  on  the  books  debited  to  the  proper  opera- 


BURNING  CLAY  WARES. 


137 


tion.  Clayworkers  would  find  it  profitable  to  analyze  the  ash 
for  carbon,  a simple  and  inexpensive  analysis — and  then 
by  tests  of  bars  and  furnaces  select  that  which  shows  the  least 
loss. 

A double  furnace  (Figs.  23  and  24)  has  proven  very  satis- 
factory in  burning  ware  where  constant  oxidizing  conditions 
are  required  and,  in  fact,  such  a furnace  is  preferable  to  a 
single  furnace  in  any  operation.  The  twin  furnaces  have  a 
single  bag,  and  they  are  fired  alternately.  When  one  is  freshly 
fired,  the  distillation  gases  are  a maximum  and  the  secondary 
air  is  insufficient  for  complete  combustion.  The  adjacent  fur- 
nace has  in  tlie  meantime  reached  the  burning  coke  stage, 
likely  with  some  excess  air.  The  products  from  both  furnaces 
mingle  more  or  less  in  the  single  bag  and  that  which  one  lacks 
in  air  and  temperature  is  supplied  by  the  other.  If  the  fur- 


Fig.  23.  Fig.  24. 


naces  are  clinkered  at  proper  intervals,  one  will  be  at  its  maxi- 
mum heating  efficiency  when  the  other  is  approaching  the 
period  of  lowest  efficiency,  followed  by  chilling  effect  of  clinker- 
ing  and  filling  with  fresh  fuel. 

Secondary  Air. 

Secondary  air  is  essential  for  complete  combustion,  and  its 
admission  to  the  furnace  has  been  the  subject  of  considerable 
study  and  numerous  patents.  A few  holes  in  the  furnace  door 
are  the  simplest  arrangements  and  likewise  the  least  satis- 
factory. Leaving  the  doors  partly  open  is  less  satisfactory, 
because  of  the  carelessness  of  the  operators.  In  such  oper- 
ation we  have  found  the  openings  varying  from  half  an  inch 
to  half  the  width  of  the  firing  hole. 


138 


BURNING  CLAY  WARES. 


Setting  the  door  to  leave  a space  between  the  bottom  of 
the  door  and  the  firing  plate  is  better,  since  it  introduces  air 
at  the  bottom,  but  being  a fixed  opening  makes  it  objection- 
able, especially  where  alternating  oxidizing  and  reducing  con- 
ditions were  required.  Leaving  a space  between  the  door 
plate  and  the  ends  of  the  grate  bars  is  still  better  (see  Fig.  24) 
in  that  it  brings  the  air  into  immediate  contact  with  the  burn- 
ing coal,  and  it  can  be  readily  closed  by  a loose  plate  or  even 
by  coal  tailing  out  on  the  door  plate. 

A number  of  patented  constructions  have  been  introduced 
which  look  good  on  paper,  some  of  which  have  merit  in  practi- 
cal use. 

Fig.  25  and  Fig.  26  illustrate  one  method  of  introducing 
hot  secondary  air,  which  has  been  extensively  used. 

The  furnace  lining  is  separated  from  the  back  wall  by  a 


Fig.  25.  Fig.  26. 


four-inch  space  and  this  space  connects  with  the  outside  air 
through  a hole  in  the  front  wall,  and  similar  holes  in  the 
furnace  lining  introduce  the  air  into  the  furnace.  The  air  is 
heated  by  the  furnace  walls  before  entering  the  furnace  and  it 
can  be  shut  off  in  any  degree  by  closing,  partially  or  entirely, 
the  opening  in  the  outside  wall. 

We  had  extended  experience  with  this  design,  but  we  do 
not  know  what  benefit  resulted.  Perhaps,  when  first  installed 
it  was  effective,  but  after  a time  when  the  furnace  doors  be- 
came warped  and  no  longer  close  fitting,  and  the  secondary 
air  ports  became  clogged  with  ash  and  clinker,  it  is  likely 
that  the  resistance  to  the  entering  air  was  less  around  the 
doors  than  through  the  air  ports,  and  that  the  secondary  air 
entered  by  this  route  rather  than  by  the  one  provided. 


BURNING  CLAY  WARES. 


139 


Another  method  which  is  more  promising  introduces  the 
air  through  the  ash  pit  walls  from  the  inside,  which  rises  in 
the  space  between  the  furnace  lining  and  wall  and  enters  a 
similar  space  between  the  inner  and  outer  crown  arches,  thence 
through  ports  into  the  bag,  as  shown  in  Fig.  27.  This  method 
has  the  advantage  over  the  preceding  one  in  that  the  upcast 
would  act  as  a stack,  and  possibly  there  would  be  some  aspi- 
rating effect  of  the  gases  rising  in  the  bag. 

Another  method,  shown  in  Fig.  28,  introduces  the  air  from 
the  ash  pit  through  the  base  of  bag  wall,  but  if  it  is  effective, 
it  is  bad  in  that  the  air  would  tend  to  cool  the  bottom  of  the 
bag  wall,  whereas  we  depend  upon  the  heat  conducted  through 
the  bottom  of  the  bag  wall  to  finish  the  ware  on  the  kiln  floor 
near  the  bag  wall. 

Several  designs  have  been  worked  out  to  use  the  heat 


i . 


Fig.  27.  Fig.  28. 


from  the  flues  under  the  downdraft  kiln  floor  by  means  of 
parallel  adjacent  air  ducts  leading  to  the  furnaces — in  one 
instance  for  the  generation  of  power  steam — but  these  efforts 
are  usually  unsatisfactory  in  that  they  tend  to  cool  the  part 
of  the  kiln  we  have  the  greatest  difficulty  in  heating  up. 

Coking  Table  Furnaces. 

The  coking  table  furnace  is  used  in  many  plants  for  bitu- 
minous fuel. 

Fig.  29  illustrates  a single  furnace  with  a double  coking 
table,  and  Fig.  30  shows  a double  furnace  with  a single  coking 
table. 

The  coal  is  first  placed  on  the  coking  table,  then  before 


140 


BURNING  CLAY  WARES. 


each  firing  the  coke  is  pushed  off  the  table  and  falls  on  the 
grates  where  the  combustion  is  completed. 

A type  with  the  coking  table  in  front  is  shown  in  Fig.  31, 
which  has  been  used  to  some  extent  in  kiln  furnaces  and  also 
in  boiler  furnaces.  This  furnace  can  be  clinkered  without 


Fig.  29. 


Fig.  30. 


opening  the  firing  door  and  secondary  air  enters  through  the 
space  between  the  coking  table  arch  and  the  grate  bars.  The 
objection  to  these  furnaces  is  that  they  involve  extra  labor 
on  the  part  of  the  fireman  and  increase  the  period  during 
which  the  firing  door  is  open.  In  firing  a flat  grate  the  fire- 
man spreads  the  coal  over  the  furnace  by  a dexterous  move- 
ment of  the  shovel,  but  to  level  the  coke  after  it  leaves  the 


Fig.  31. 


Fig.  32. 


table  requires  a separate  operation  before  again  charging  the 
coking  table. 

The  McManigal  furnace,  shown  in  Fig.  32,  combines  a 
coking  table  with  the  pit  type  of  furnace,  which  simplifies  the 
operation  in  that  the  pit  furnace  requires  no  leveling.  Sec- 


BURNING  CLAY  WARES. 


141 


ondary  air  in  the  natural  draft  furnaces  is  admitted  through 
small  horizontal  ducts  in  the  coking  table  as  shown  by  the 
dotted  line. 


Pit  Furnaces. 

A pit  furnace  is  in  effect  a gas  producer.  The  most  com- 
mon form  is  the  inclined  grate  bar,  illustrated  in  Fig.  33, 
which  resembles  the  original  Siemens  producer. 

The  grates  are  usually  flat  bars  of  iron  hooked  over  a 
bearing  bar  at  the  mouth  of  the  furnace  and  extending  to 
within  about  one  foot  of  the  pit  floor. 

The  bars  are  set  at  an  angle  varying  from  30  to  60  degrees 


Figure  33. 


from  the  vertical,  and  their  purpose  is  less  that  of  grate  bars 
than  that  of  a supporting  plate. 

The  coal  is  fired  at  the  top,  and  the  secondary  air  supply 
is  regulated  by  partially  or  completely  closing  the  fire  mouth 
with  coal.  As  the  coke  in  the  pit  burns  and  its  volume  be- 
comes reduced,  the  coal  on  the  plate  slips  downward  or  is 
forced  down  with  the  shovel  prior  to  introducing  a fresh  sup- 
ply of  coal.  Thus  the  coal  passes  through  the  several  pro- 
ducer zones — distillation,  dissociation,  combustion  and  ash. 

The  primary  air  largely  enters  through  the  ash  tailing  out 
below  the  grate  bars.  The  bed  of  coal  is  necessarily  deep, 
and  the  gas  rising  from  it  is  lean  producer  gas  from  coke. 
This  mixes  with  the  distillate  gas  and  secondary  air  in  the 
combustion  space  over  the  bed  of  coal  where  the  final  com- 
bustion takes  place. 

The  usual  construction  of  the  furnace  is  with  a straight, 
horizontal  arch,  but  Hull  in  Trans.  A.  C S.  shows  the  advan- 


142 


BURNING  CLAY  WARES. 


tage  of  stepping  down  the  arch  toward  the  bag,  as  shown  by 
the  dotted  lines  in  Fig.  33.  This,  or  a simple  drop  arch  on  the 
inner  ring,  forces  the  air  downward  keeping  it  in  close  touch 
with  the  coal  and  insures  better  mixing  of  the  air  and  gases. 

It  is  not  essential  that  we  get  the  distinctive  results  of  a 
producer  since  the  furnace  is  a part  of  the  kiln  and  we  get  all 
the  heat  of  the  combustion  except  that  lost  by  radiation  from 
the  front,  regardless  of  the  particular  zone  in  which  the  heat 
is  developed.  There  are  many  modifications  of  this  simple 
furnace,  some  good  and  some  not  so  good. 

Frequently  a solid  plate  of  brick  work  or  fire  clay  blocks  is 
used  instead  of  the  grate  bars  and  our  preference  is  for  such 
a solid  plate  as  shown  in  Fig.  34  and  Fig.  35.  It  enables  us 


Fig.  34.  Fig.  35. 


to  reduce  the  size  of  the  furnace  mouth  and  gives  us  better 
control  of  the  secondary  air. 

The  Paul  Beers  furnace  is  illustrated  in  Fig.  36  and  needs 
no  explanation,  except  to  call  attention  to  the  secondary  air 
port  in  the  drop  arch,  which,  since  it  can  be  readily  closed, 
gives  the  burner  better  control. 

The  McManigal  furnace,  Fig.  32,  as  noted,  has  a coking 
table,  and  this  table  is  the  distillation  zone. 

McManigal  has  also  introduced  a forced  draft  feature,  and 
the  air  for  primary  combustion  is  heated  by  the  radiation  from 
the  kiln  crown.  A second  crown  is  sprung  over  the  first  with 
a space  between  the  two.  Air,  by  means  of  a fan,  is  forced 
through  the  top  crown  vent  into  the  space  between  the  two 
crowns,  and  thence  at  the  spring  level  is  conducted  through 
pipes  to  the  furnaces  and  introduced  into  the  ash  pit,  which 
is  provided  with  a tight  door.  See  Fig.  37.  In  this  way  he 


BURNING  CLAY  WARES. 


143 


collects  the  heat  loss  by  radiation  through  the  crown  and 
returns  it  to  the  furnaces. 

Unclassified  Furnaces. 

The  Boss  system  of  burning  is  a forced  draft  application. 
The  furnace  is  simply  an  arched  rectangular  opening  in  the 
kiln  wall.  On  the  ground  level  in  the  furnace  is  placed  a flat, 
rectangular  cast-iron  box  about  12  inches  by  24  inches  with 
perforated  top,  or  more  correctly,  with  numerous  air  ports  of 
special  design  in  the  top.  One  end  of  the  casting  has  a con- 
nection for  the  blast  pipe.  The  advantage  is  that  slack  coal 
or  slack  and  nut  coal  can  be  burned  at  a rapid  rate,  whereas 
it  would  not  be  possible  to  use  such  fuel  in  a natural  draft 
furnace. 

The  following  data  relative  to  the  Boss  system  is  an  ab- 


Fig.  36. 


stract  from  the  Transactions  of  the  National  Brick  Manufac- 
turers’ Association. 

Subject — “Forced  Draft  in  Up-draft  Kilns.” 

“The  product  is  common  bricks  burned  in  updraft  kilns. 
The  kilns  are  sixteen  arches  long  and  21  feet  wide,  and  the 
bricks  are  set  42  courses  high  in  the  usual  manner  and  platted. 

The  burners  (grates)  are  six  inches  wide  and  two  feet 
long,  and  each  burner  has  14  spaces  5^  inches  long  and  %-inch 
wide  through  which  the  air  passes.  The  burners  are  placed 
in  the  fire  box  level  with  the  kiln  floor. 

The  fan  is  41/£  feet  in  diameter  and  is  driven  by  a six  or 
eight-horse  power  engine. 

A 24-inch  duct  runs  at  right  angles  to  the  kilns  and  there 


144 


BURNING  CLAY  WARES. 


is  a 12-inch  duct  down  the  side  of  each  kiln,  and  from  these 
side  ducts  3-inch  pipes  lead  to  the  furnaces.  The  draft  is 
regulated  by  the  speed  of  the  fan  and  is  measured  by  a “TJ” 
tube  water  gage. 

The  fan  suffices  for  four  kilns. 

In  starting  a kiln  the  pressure  is  put  on,  then  a shovelful 
of  coals  is  thrown  into  each  firehole,  followed  by  a shovelful 
of  slack  coal.  The  initial  pressure  is  one  inch  to  1%  inches. 

During  the  water  smoking  the  fire  doors  are  left  open  and 


the  platting  on  top  is  up.  When  the  dampness  is  off  the 
pressure  is  increased  to  1%  inches  to  2 inches  and  the  firing 
consists  of  one-half  a small  shovelful  of  slack  coal  in  each  hole 
at  8-minute  intervals.  In  about  two  days  when  the  fire  is 
through  the  bricks,  the  platting  is  tightened.  The  firing  then 
is  about  10-minute  intervals  using  the  same  amount  of  coal. 
The  total  time  required  is  six  or  seven  days,  during  which 
six  to  seven  bushels  of  coal  per  thousand  bricks  are  used, 
with  a result  of  80  to  85  per  cent,  hard  bricks.” 

The  advantages  claimed  for  any  pressure  system  are:  (1) 


BURNING  CLAY  WARES. 


145 


More  rapid  combustion,  quicker  burns  in  consequence,  thus 
reducing  the  gross  kiln  radiation  loss.  (2)  Pressure  distri- 
bution of  the  hot  gases  throughout  the  kiln,  thus  getting  more 
uniform  results. 

A step-grate  furnace,  Fig.  38,  combines  a grate-bar  furnace 
in  maximum  degree  with  a pit  furnace  in  limited  degree.  The 
purpose  is  to  get  an  excessive  bar  surface  within  a limited 
space  and  at  the  same  time  get  some  degree  of  a producer 
condition  to  develop  unconsumed  gases  to  take  up  the  excess 
air  coming  through  the  bar  spaces.  This  type  of  furnace  is 
particularly  useful  in  burning  low-grade  fuels,  such  as  lignite. 

A number  of  furnaces,  which  carry  out  the  producer  gas 


Fig.  38. 


principle,  have  been  designed,  but  they  are  seldom  used  in 
this  country  in  clayware  burning. 

Position  and  Size  of  Furnaces. 

The  possibility  of  burning  clay  wares  economically  depends 
largely  upon  the  furnace  power  of  the  kiln,  and  in  this  im- 
portant item  engineers  differ  widely.  We  have  seen  twelve 
furnaces  in  a 30-foot  diameter  round  kiln  to  burn  ware  to  cone 
08 ; eighteen  furnaces  in  an  equivalent  kiln  for  cone  1 tem- 
peratures ; twelve  furnaces  for  cone  7 to  10 ; ten  furnaces  for 
cone  1,  also  for  cone  9 ; ten  to  twelve  furnaces  for  cone  20 ; 
eight  furnaces  in  a 37-foot  kiln  to  cone  18.  One  authority, 
whose  opinion  we  value,  states  that  if  it  were  possible  he 
would  have  a ring  of  fire  entirely  around  the  kiln  and  at  the 
level  of  the  top  of  the  bag.  Another  authority  would  have 
the  grate  level  four  or  more  feet  below  the  kiln  floor  level  if 
practicable. 


146 


BURNING  CLAY  WARES. 


The  grate  level  should  not  be  above  the  kiln  floor  level  if 
for  no  other  reason  than  that  leakage  from  the  ash  pit  through 
the  base  of  the  bag  wall  will  keep  the  temperature  in  this 
part  of  the  kiln  below  the  required  temperature  resulting  in 
soft  ware  around  the  bottom  of  the  bags.  We  believe  there 
is  an  advantage  in  the  burning  by  placing  the  grate  level  be- 
low the  kiln  floor  level,  but  the  advantage  is  less  than  the  dis- 
advantage of  the  depressed  firing  pit.  A grateless  furnace 
gives  a level  yard  throughout  and  we  would  not  go  below  this 
level  with  this  type  of  furnace. 

A flat-bar  furnace,  or  any  furnace  with  separate  ash  pit, 
requires  a depressed  firing  pit  to  get  the  grate  bars  down  to 
the  kiln  floor  level,  and  we  may  readily  go  6 inches  to  12 
inches  deeper  to  get  the  bar  level  below  the  floor  level. 

Furnaces  should  not  be  wider  than  36  inches  nor  longer 
than  48  inches  in  order  to  clean  them  readily.  It  is  better 
to  increase  the  number  of  furnaces  rather  than  the  size. 

Grateless  furnaces  vary  in  width  from  20  inches  to  30 
inches,  depending  upon  the  coal.  If  the  coal  is  high  in  ash 
and  clinkers  badly,  the  wider  furnace  should  be  used  in  order 
to  insure  sufficient  free  area  for  the  required  combustion  air. 

A number  of  years  ago,  after  investigation  of  many  kiln 
operations,  we  adopted  the  rule  that  for  cone  1 the  flat  grate 
area  should  be  1.5  per  cent,  of  the  cubical  setting  space  in 
the  kilns,  with  an  increase  of  .05  per  cent,  for  each  additional 
cone.  Pit  furnaces  which  get  air  through  the  pit  area  and 
which  are  not  restricted  by  grate  bars  should  be  about  two- 
thirds  the  size  of  the  flat-bar  furnaces.  In  designing  a fur- 
nace we  should  always  take  into  consideration  the  character 
of  the  fuel,  the  kiln  tonnage  and  the  time  required  to  burn. 

A practical  rate  of  combustion  is  eight  pounds  of  coal  per 
foot  of  flat  grate  area  per  hour.  This  may  be  increased  to 
twelve  pounds  in  burning  hollow  ware,  or  other  ware  not 
closely  set  as  bricks,  in  consequence  of  lessened  kiln  resist- 
ance applicable  to  overcome  the  furnace  resistance,  but  in 
burning  closely  set  ware  the  rate  is  often  less  than  eight 
pounds. 

As  a basis  for  such  determination  the  following  general 
data  for  bituminous  coal  per  ton  of  burned  ware  may  be  used : 


Fuel.  Time. 

Common  bricks 150  to  400  lbs.  of  coal  5 to  8 days 

Face  and  sewer  bricks 300  to  700  lbs.  of  coal  6 to  12  days 

Paving  blocks  500  to  800  lbs.  of  coal  6 to  12  days 

Drain  tile 350  to  800  lbs.  of  coal  2 to  5 days 

Hollow  blocks  300  to  600  lbs.  of  coal  3 to  5 days 

Sewer  pipe 800  to  1,500  lbs.  of  coal  5 to  8 days 

Terra  cotta  1,200  to  1,800  lbs.  of  coal  3 to  5 days 


BURNING  CLAY  WARES. 


147 


These  requirements  are  on  the  assumption  of  an  average 
quality  of  coal,  and  the  differences  are  due  to  variations  in 
temperature  required,  to  furnace  and  kiln  construction  and  to 
the  care  taken  in  the  firing  operation. 

A dirty,  high  ash,  badly  clinkering  coal,  will  require  a 
larger  grate  area  in  order  to  maintain  a proper  free  area  and 
similarly,  a low  value  lignite  must  have  provision  for  burning 
two  or  three  times  the  estimated  quality.  The  tonnage  and 
character  of  the  setting  must  also  receive  some  consideration 
as  well  as  the  time  required  to  burn.  Some  kilns  are  stacked 
to  the  crown  with  ware,  while  other  similar  ware  requires 
lower  setting. 

A kiln  set  with  90  tons  of  drain  tile  will  require  larger 
furnaces  than  one  set  with  60  tons.  Paving  blocks  in  some 
instances  are  burned  in  four  and  one-half  to  five  days,  but  in 
others  require  ten  to  thirteen  days  with  little  difference  in 
temperature  requirement.  Obviously,  the  former  should  have 
greater  grate  area. 

The  question  might  arise,  why  not  increase  the  stack  height 
and  burn  at  a faster  rate,  to  which  it  may  be  replied  that 
such  high  rate  of  combustion,  reducing  as  it  would  the  per 
cent,  radiation  loss,  would  give  a higher  flame  temperature  in 
the  top  of  the  kiln  and  in  consequence  a rapid  absorption  of 
heat  by  the  top  ware  to  its  ruin. 

When  we  have  attained  the  finishing  temperature  on  top 
we  simply  wish  to  hold  it  by  gases  of  approximately  the  same 
temperature,  and  work  this  temperature  to  the  bottom  of  the 
kiln.  We  could  get  the  desired  bottom  temperature  quicker 
with  a higher  rate  of  combustion,  but  to  do  so  would  over- 
burn the  top,  perhaps  ruin  it,  and  in  any  event  the  result  from 
top  to  bottom  would  be  less  uniform. 

To  get  uniform  results  in  a down  draft  kiln  we  must  work 
with  a so-called  balanced  draft,  namely,  a draft  which  will 
merely  keep  the  furnaces  cleared  of  their  gases  and  which  if 
ever  so  slightly  reduced  will  cause  the  furnaces  to  smoke 
around  the  feed  holes.  This  applies  particularly  to  the  later 
stages  of  the  burning  when  the  ware  on  top  has  attained  the 
finishing  temperature. 

During  the  early  stages  of  burning — water  smoking  and 
heating  up — when  the  stack  intensity  is  low,  we  should  use 
all  the  draft  we  can  get,  and  in  the  majority  of  instances  more 
would  oe  desirable. 

Greaves-Walker  bases  the  grate  area  on  the  kiln  area. 


148 


BURNING  CLAY  WARES. 


His  maximum  furnace  has  one  foot  of  grate  area  to  four 
feet  of  floor  area  and  the  minimum  has  one  foot  of  grate  to 
eight  feet  of  floor.  He  adopts  one  foot  of  grate  to  7.5  feet  of 
floor  as  the  best  average  ratio.  For  sewer  pipe  and  similar 
salt-glazed  ware  he  gives  a ratio  between  1 to  6 and  1 to  8, 
the  latter  having  the  preference. 

If  the  furnace  area  is  too  small,  there  will  be  an  excessive 
wnste  of  fuel: 

(1)  Because  of  a required  longer  firing  period. 

(2)  Because’ of  frequent  stirring  of  the  fires  and  repeated 
clinkering  to  get  a necessary  higher  rate  of  combustion  re- 
sulting in  a higher  carbon  content  in  the  ash. 

An  excess  grate  area  within  reasonable  limits  can  be  con- 
trolled to  a minimum  rate  of  combustion  by  longer  intervals 
between  firing  periods  and  by  allowing  the  clinker  to  accumu- 
late and  thus  reduce  the  free  area. 

Comparison  of  Furnace  Areas. 

A 30-foot  round  down-draft  kiln  has  about  650  feet  of 
floor  area,  exclusive  of  the  bag  walls.  An  average  setting 
height  of  9 feet  will  give  5,850  cubic  feet.  On  the  basis  of  the 
first  rule,  the  area  of  grate  surface  of  such  a kiln  for  cone 
1 ware  will  be  8.8  square  feet,  or  nine  furnaces  of  10  square 
feet  area  each,  reducing  to  eight  furnaces  for  temperatures 
below  cone  1.  Cone  5 temperature  ware  will  require  ten  fur- 
naces ; cone  10  temperature,  twelve  furnaces ; cone  26  tempera- 
ture, sixteen  furnaces. 

A ratio  of  one  foot  of  grate  area  to  four  feet  of  floor  area 
would  require  sixteen  furnaces,  and  a ratio  of  1 to  8 would 
require  eight  furnaces. 

Either  of  these  rules  in  the  hands  of  a competent  engineer, 
who  will  take  into  consideration  the  kiln  tonnage,  time  re- 
quired to  burn,  and  the  quantity  of  fuel  required  per  ton,  will 
give  a kiln  and  furnace  design  well  within  reasonable  limits 
of  a practical  and  economical  rate  of  combustion. 

Let  us  consider  furnace  areas  for  a 30-foot  round  kiln  on 
the  basis  of  fuel  and  time  given  on  a preceding  page.  We  will 
assume  20  per  cent,  of  the  time  is  required  for  the  water- 
smoking, during  which  time  the  rate  of  combustion  is  slow, 
and  we  will  make  no  allowance  for  the  fuel  used  during  this 
period. 

The  results  are  as  follows: 


BURNING  CLAY  WARES.  149 


Common  Brick- 

— 240  Tons. 

No.  of 

Coal  Time 

Grate  Area 

Furnaces 

150  lbs.  coal,  4 days 

5 

400  lbs.  coal,  4 days 

12 

400  lbs.  coal,  6.4  days 

8 

Average 

8 

Face  Brick — 

210  Tons. 

300  lbs.  coal,  4.8  days 

7 

700  lbs.  coal,  4.8  days 

16 

700  lbs.  coal,  9.6  days 

8 

Average 

. .103  sq.  ft. 

10 

Paving  Blocks- 

— 200  Tons. 

500  lbs.  coal,  4.8  days 

. .109  sq.  ft. 

11 

800  lbs.  coal,  4.8  days 

. . 173  sq.  ft. 

17 

800  lbs.  coal,  9.6  days 

, . . 87  sq.  ft. 

9 

Average 

12 

Drain  Tile — 75  Tons. 

350  lbs.  coal,  1.6  days 

. . 53  sq.  ft. 

5 

800  lbs.  coal,  1.6  days 

19 

800  lbs.  coal,  4 days 

8 

Average 

11 

Sewer  Pipe — 60  Tons. 

800  lbs.  coal,  4 days 

6 

1500  lbs.  coal,  4 days 

, . . 117  sq.  ft. 

12 

1500  lbs.  coal,  6.4  days 

, ..  73  sq.  ft. 

7 

Average 84  sq.  ft.  8 


The  drain  tile  estimate  is  open  to  criticism  because  only 
small  kilns  can  be  burned  off  in  two  days,  and  in  no  case  would 
the  maximum  quantity  of  fuel  be  burned  in  this  short  period  of 
time,  and  the  other  intermediate  estimates  are  open  to  the 
same  criticism,  but  in  less  degree.  These  estimates  bring  out 
two  facts: 

(1)  The  combustion  rate  of  8 pounds  of  coal  per  square 
foot  of  grate  area  per  hour  is  a maximum  for  average  prac- 
tice, because  if  we  were  to  figure  a higher  rate  of  combustion, 
the  minimum-fuel-minimum-time  operation  would  require  an 
absurdly  small  grate  area  not  found  in  practice  anywhere. 
The  maximum-fuel-maximum-time  operation  gives  results  ap- 


150 


BURNING  CLAY  WARES. 


proximating  general  practice,  being  under,  however,  rather 
than  over;  and  if  we  were  to  increase  the  rate  of  combus- 
tion 50  per  cent,  the  grate  area  would  be  less  than  that  we 
know  to  be  practical.  The  intermediate  figures  would  indi- 
cate a higher  rate  of  combustion,  but  it  seldom  happens  that 
the  maximum  quantity  of  fuel  is  burned  in  the  minimum 
period  of  time  upon  which  assumption  these  intermediate 
figures  are  based. 

(2)  The  rules  given  for  grate  areas  will  give  the  proper 
furnace  power  if  one  will  take  into  consideration  any  unusual 
conditions  and  make  allowance  for  them. 

Furnace  Doors. 

The  furnace  door  is  the  most  important  feature  of  the  fur- 
nace, because  it  is  the  only  movable  feature  and  easily  gets 
out  of  shape,  with  the  result  that  large  volumes  of  air  are 
being  drawn  into  the  furnace  around  the  door,  and  of  this 
we  have  no  control.  An  ill-fitting  door  wastes  in  fuel  each 
burn  the  cost  of  two  good  doors. 

A furnace  of  the  inclined  grate  bar  type  without  a door  is 
preferable  to  a door  which  does  not  fulfill  its  purpose.  The 
former  we  control  by  banking  up  the  coal,  and  if  the  com- 
bustion gases  are  investigated  to  determine  the  most  eco- 
nomical firing  conditions,  the  fire  mouths  can  be  banked  to 
maintain  this  condition. 

Secondary  air  is  necessary,  but  it  is  important  that  we 
have  it  under  control,  which  we  cannot  have  through  an  irregu- 
lar opening  around  a badly  warped  door  or  door  frame. 

Terra  cotta,  pottery  and  abrasive  products  kilns  all  under 
roof  and  enclosed  generally  use  a substantial  cast  iron  frame 
and  swinging  door  similar  to  a boiler  furnace  door,  the  door 
being  lined  with  a cast  iron  perforated  shield  set  away  from 
the  door,  or  a fire  clay  block  may  be  used. 

Such  doors  are  not  satisfactory  for  outside  kilns  exposed 
to  all  atmospheric  conditions,  but  it  must  be  mentioned  that 
the  doors  used  in  outside  kilns  are  frequently  much  less  sub- 
stantial than  the  doors  used  on  the  above  mentioned  kilns. 
Whether  the  trouble  is  due  to  a flimsy  door  equipment  or  to 
severe  conditions,  the  fact  remains  that  cast  iron  swing  doors 
have  been  largely  abandoned  in  the  construction  of  outside 
kilns,  except  in  scoved  and  up-draft  kilns. 

We  frequently  see  thin  sheet  iron  plates  used  for  furnace 
doors,  which  are  in  a condition  to  discredit  a scrap  pile  and 
which  attached  to  a kiln  are  wasting  the  profits  of  the  busi- 


ness. 


BURNING  CLAY  WARES. 


151 


A cast  iron  plate,  ribbed  to  reduce  warpage,  and  grooved 
on  the  bottom  or  with  lugs  to  slide  on  a projecting  angle  plate 
makes  a fairly  good  door.  The  front  of  the  furnace  wall  is 
battered  so  the  plate  retains  its  place  by  its  own  weight. 

A good  door  is  made  of  a shallow  cast  iron  box  lined  with 
thin  fire  clay  brick.  This  is  hung  by  a trolley  to  the  top  wall 
of  the  furnace  front,  or  it  may  be  hung  by  a wire  attached 
to  a hook  in  the  kiln  wall  higher  up,  and  projecting  from  the 
wall  just  enough  to  clear  the  front  wall  of  the  furnace.  Simi- 
larly brick  or  a clay  block  may  be  bound  with  an  iron  band 
and  hung  on  a trolley  or  by  a wire  as  above  mentioned.  A 
large  fireclay  slab  bound  with  an  iron  band  to  be  raised  and 
lowered  by  a chain  or  flexible  steel  rope,  using  a grooved 
pulley  and  counter-balanced,  is  an  excellent  door.  This  is 
the  type  of  door  used  in  heating  furnaces. 

Construction  of  Furnace. 

The  furnace  lining  should  be  built  of  the  best  quality  of 
fire  brick  laid  in  fire  clay  mortar  of  the  same  quality  as  the 
brick,  and  the  bed  joint  should  be  as  thin  as  it  is  possible  to 
make  it.  The  wall  should  be  not  less  than  9 inches  thick  and 
built  independent  of  the  kiln  wall. 

It  is  customary  in  building  the  main  kiln  wall  to  complete 
and  bond  in  the  furnace  wall  as  part  of  the  main  wall,  par- 
ticularly the  main  wall  lining  and  the  furnace  lining.  This  is 
essential  in  order  to  turn  the  inner  furnace  crown  satisfac- 
torily. The  furnace  walls  may  be  carried  up  with  the  main 
walls,  and  crowned  without  bonding  the  former  into  the  latter. 
With  such  a construction,  when  repairs  become  necessary  the 
furnace  walls  may  be  removed  and  rebuilt  without  tearing 
an  irregular,  jagged  hole  in  the  main  wall,  which  is  difficult 
to  fill  up  in  a substantial  manner. 

The  furnace  inner  crown  should  be  a single  course  of  arch 
brick  and  separated  from  the  outer  supporting  crown  by  an 
expansion  space.  A 9-inch  crown  of  wedge  brick  or  a bonded 
arch  has  the  objection  that  the  inner  ends  of  the  wedge  brick 
or  the  inner  course  of  the  bonded  arch  will  shrink  under  the 
intense  heat,  and  the  inner  points  will  break  off,  or  the  inner 
courses  slip  down  and  out,  to  the  destruction  of  the  crown. 
The  outer  arch  is  the  kiln  wall  support,  and  there  should  be 
no  occasion  to  remove  it  in  making  furnace  repairs. 

More  substantial  arches  can  be  built  of  special  shapes, 
using  two  or  at  most  three  blocks  for  the  skew  and  arch.  A 
single  brick  rowlock  arch  over  the  special  block  arch  will  sup- 


152 


BURNING  CLAY  WARES. 


port  the  main  wall  and  provide  vertical  expansion  space  for  the 
furnace  walls  and  crown. 

When  furnaces  project  from  the  main  wall,  they  should 
be  held  in  place  by  a band  or  channel  plate. 

Round  kilns  are  usually  built  with  a hub  which  includes 
the  furnace.  The  furnaces  thus  are  prevented  from  spread- 
ing by  the  kiln  walls,  but  in  addition  there  should  be  a band 
at  the  grate  bar  level. 

Projecting  furnaces  are  frequently  seen  in  rectangular 
kilns,  and  they  may  readily  be  held  in  place  by  a channel 
plate  at  the  grate  bar  level,  extending  the  full  length  of  the 
kiln.  Between  the  furnaces,  stay  rods  connecting  the  channel 
to  the  main  vertical  kiln  buck  stays  hold  the  channel  in  place 
and  thus  brace  the  furnace  wall. 

It  is  a good  plan  to  batter  the  furnace  front,  which  reduces 
the  tendency  to  draw  off,  and  also  the  doors  will  hug  the  wall 
instead  of  hanging  away  from  it  as  happens  when  the  wall 
overhangs. 


BURNING  CLAY  WARES. 


153 


CHAPTER  VIII. 

KILNS. 

KILNS  MAY  BE  classified  as  follows,  and  while  the 
classification  given  below  is  somewhat  incongrous 
and  open  to  criticsm,  still  it  has  the  merit  of  arrang- 
ing the  kilns  in  some  relation  to  the  wares  produced. 

I.  Open-top  kilns. 

(1)  Periodic  in  operation. 

(2)  Continuous  in  operation. 

(A)  Ring  kilns. 

( B ) Chambered  kilns. 

II.  Crowned  kilns. 

(1)  Open  fire. 

(A)  Periodic  in  operation. 

(a)  Up  draft 

(b)  Down  draft. 

(c)  Up  and  down  draft. 

(d)  Horizontal  draft. 

(B)  Continuous  in  operation. 

(a)  Ring  kilns — horizontal  draft. 

(b)  Chambered  kilns — down  draft. 

(c)  Car  tunnel  kilns. 

Horizontal  draft. 

Down  draft. 

(2)  Muffled  kilns. 

(A)  Periodic  in  operation. 

(B)  Continuous  (car  tunnel)  in  operation. 

The  first  general  division  applies  to  common  bricks,  hollow 
bricks  or  blocks,  and  drain  tile  in  small  degree. 

The  second  general  division  includes  all  other  types  of 
kilns  and  the  first  subdivision  covers  a broad  field  and  in- 
cludes every  line  of  ware  from  common  bricks  to  poreclain. 
The  white  wares,  however,  are  inclosed  in  saggers  which  are 
multiple  muffles,  and  we  might  properly  duplicate  several  of 


154 


BURNING  CLAY  WARES. 


the  types  in  the  first  subdivision  under  muffled  kilns,  which 
would  leave  the  first  division  applicable  to  common  wares — 
bricks  of  all  kinds,  fire-proofing  and  other  unglazed  hollow 
ware.  It  would  also  include  salt-glazed  wares  and  slip-glazed 
ware  such  as  stoneware — in  fact,  any  ware  that  can  be  burned 
in  contact  with  the  fire  gases. 

The  second  subdivision,  then,  would  cover  terra  cotta  and 
other  sensitive  glazed  wares,  white  ware,  art  pottery,  etc. 

Periodic  Open-top  Kilns. 

The  rectangular  open-top  periodic  kilns,  known  as  “scove,” 
“clamp”  and  “up-draft”  kilns,  are  suitable  for  common  bricks 
only.  It  was  in  such  kilns  that  the  ancient  clayworkers 
burned  their  brick  hard,  as  they  were  commanded  to  do,  and 
practically  the  same  kiln  is  used  today  in  the  great  centers 
of  the  common  brick  industry. 

The  term  scoved  kiln  is  applied  to  the  mass  of  bricks  in- 
stead of  to  the  structure  inclosing  the  bricks,  in  that  we  indi- 
cate the  location  as  the  “kiln  yard”  or  “kiln  shed”  or  “kiln 
of  bricks.”  Indeed,  the  scoved  kiln  hardly  justifies  a name 
being  given  to  it  as  a kiln  structure. 

The  bricks  are  set  in  rectangular  form,  with  fire  arches, 
etc.,  and  then  cased  with  bricks,  sometimes  unburned  bricks, 
and  the  casing  daubed  with  mud. 

When  the  kiln  is  burned,  cooled,  and  shipping  begins,  the 
casing  is  thrown  down,  or  sometimes  loaded  out  with  the 
burned  product,  and  the  ground  is  cleared  for  a second  kiln. 

In  some  instances,  especially  where  coal  is  the  fuel  and 
grate  bars  are  required  in  consequence,  the  casing  wall  is 
made  permanent  to  a height  wThich  includes  the  fire  mouths 
and  arches,  and  the  usual  casing  starts  on  top  of  this  wall. 

The  ordinary  casing  is  eight  inches  thick  up  to  two-thirds 
of  the  height  of  the  setting,  then  drops  off  to  four  inches. 
Frequently  the  bottom  of  the  casing  wall  is  twelve  inches 
thick  to  the  top  of  the  fire  mouths,  then  drops  off  to  eight 
inches  and  finally  to  four  inches.  The  casings  are  laid  up  dry, 
then  daubed  to  prevent  or  reduce  the  air  leakage.  When  the 
kiln  is  finished  and  ready  to  fire,  the  top  is  completely  platted 
with  bricks  laid  flat. 

The  bricks  in  the  first  course  of  platting  are  placed  tightly 
end  to  end,  but  the  rows  are  spaced  so  that  each  course  of  the 
top  platting,  with  its  bricks  at  right  angles  to  those  in  the 
first  course,  will  center  on  the  rows  in  the  under  course. 


BURNING  CLAY  WARES. 


155 


After  the  platting  is  in  place,  or  as  it  is  placed,  each  alternate 
third  or  fourth  brick  is  raised  to  provide  draft.  For  conven- 
ience in  lifting  the  platting,  and  particularly  in  replacing 
(“tightening”)  it  after  the  “heat  is  through”  to  the  top,  it  is 
an  advantage  to  set  on  edge  each  alternate  brick  in  the  top 
course  which  gives  a finger  hold  on  the  projecting  edge.  Usu- 
ally, however,  the  alternate  loosened  bricks  are  not  lifted  out, 
but  instead  one  end  is  raised  and  the  brick  slipped  endwise, 
and  it  can  be  hooked  back  into  place  when  the  time  comes  to 
tighten  the  platting. 

The  cost  of  casing  and  platting  exclusive  of  the  bricks  used 
varies  from  $0.15  to  $0.30  per  thousand  bricks  set  in  the  kiln. 
This  is  a serious  item  in  the  cost  of  the  product,  and  the  ques- 
tion may  be  asked : How  can  such  kilns  compete  with  other 

types  of  kilns? 

The  cost  of  installation  is  very  small  in  comparison  with 
other  kilns,  thus  eliminating  interest,  depreciation  and  up- 
keep, and  the  capacity  per  kiln  is  unlimited,  varying  from 
100,000  or  less  to  a million  or  more  bricks,  usually  ranging 
between  250,000  and  700,000  bricks.  They  are  more  economical 
in  fuel  than  other  types  of  periodic  kilns. 

The  kilns  are  placed  alongside  the  loading  tracks  or  docks, 
giving  short  runs  for  loading,  especially  in  view  of  the  fact 
that  the  bricks  may  be  taken  from  any  point  along  the  kiln 
to  the  car  or  boat  placed  opposite  such  point,  whereas  in  per- 
manent kiln  structures  the  route  must  be  through  the  kiln 
entrance. 

The  kilns  being  open  and  rectangular,  the  setting  and  draw- 
ing can  be  done  at  a minimum  cost. 

Finally,  because  of  the  small  cost  of  the  kiln — in  fact,  noth- 
ing more  than  the  cost  of  any  storage  space — the  bricks  may 
remain  in  the  kiln  until  they  are  sold,  thus  saving  a double 
handling. 

The  bricks  are  set  in  benches,  the  legs  of  which  have  a 
width  of  2 y2  bricks  (21  inches)  or  3 bricks  (26  inches)  or  3 y2 
bricks  or  4 bricks,  with  a space  of  12  to  16  inches  between 
the  legs  for  the  fire  arch.  The  height  of  the  setting  is  com- 
monly 42  courses  of  bricks  on  edge,  but  in  some  instances  54 
courses  are  the  setting  height.  The  former  height  is  estab- 
lished by  the  ability  of  a man  (tosser)  to  toss  the  bricks  in 
units  of  4 or  5 bricks  to  the  setter.  Higher  setting  requires 
an  extra  man  standing  on  a lower  bench  to  receive  and  toss 
the  bricks  to  the  setter  working  on  an  upper  bench.  The  av- 


156 


BURNING  CLAY  WARES. 


erage  3-brick  bench  kilns,  set  42  courses  high,  hold  about 
20,000  bricks  in  each  bench,  varying  from  15,000  to  30,000, 
depending  upon  the  width  of  the  kiln,  and  higher  setting  will 
increase  the  quantity  correspondingly. 

The  length  of  the  kiln  is  simply  a matter  of  convenience 
or  yard  arrangement  and  is  designated  as  so  many  benches, 
arches  or  “eyes.” 

In  the  large  common-brick  centers  the  kiln  shed  is  a thou- 
sand or  more  feet  long.  The  kilns  are  set  longitudinally  in 
the  shed  and  the  coal  and  shipping  track  are  parallel  to  the 
kiln  on  each  side  either  under  the  shed  or  just  outside. 

Where  the  shipping  is  by  water,  the  kiln  shed  and  kiln  are 
close  to  the  dock  and  parallel  to  it. 

The  setting  begins  at  one  end  of  the  shed,  although  not 
essential,  and  depends  upon  whether  the  old  stock  has  been 
moved.  As  soon  as  sufficient  benches  to  constitute  an  econom- 
ical firing  unit  (in  one  district  four  arches  are  the  unit  for 
one  burner,  day  and  night,  in  another  five  arches)  are  set, 
the  end  of  the  kiln  is  closed,  cased  and  daubed,  and  the  fires 
started.  The  sides  of  the  kiln  are  cased  as  the  setting  pro- 
gresses. 

A second,  third,  etc.,  kiln  continues  the  setting  to  the  other 
end  of  the  shed.  Meanwhile  the  first  kiln  will  be  burned, 
cooled  and  loaded  out,  and  the  setting  returns  to  the  original 
starting  point. 

If  the  market  is  good  the  shed  will  be  kept  clear  for  the 
setting  until  late  in  the  fall,  in  some  instances  throughout 
the  winter  where  winter  work  is  possible.  If  the  bricks  are 
not  moved,  the  season  closes  when  the  setting  overtakes  the 
drawing  and  the  shed  is  full.  This  is  the  common-brick  con- 
tinuous kiln  operation,  but  the  individual  kiln  is  periodic 
and  not  a continuous  kiln,  as  we  ordinarily  and  properly  des- 
ignate continuous  kilns. 

The  arrangement  of  the  kilns  depends  upon  the  layout  of 
the  yard.  Sometimes  the  kilns  are  endwise  to  the  loading 
track  on  one  side  and  to  the  factory  on  the  the  other  side,  and 
some  plants — partly  rail  shipping  and  partly  direct  delivery 
by  wagons,  or  all  wagon  delivery — have  the  kilns  distributed 
around  the  factory  on  two  or  three  sides. 

If  scoved  kilns  or  the  permanent  wall  up-draft  kilns  pro- 
duced as  good  a product  as  other  types  of  kilns,  they  would 
find  a wider  use  and  would  be  difficult  to  displace.  Unfortu- 
nately, they  do  not  give  the  results.  The  arch  bricks,  which 


BURNING  CLAY  WARES. 


157 


in  effect  are  a continuation  of  the  furnace  walls,  are  checked, 
spalled  and  blackened  by  the  fire  on  the  end  exposed  to  the 
fire,  and  where  the  shrinkage  is  considerable  they  are  wedge- 
shaped.  The  top  bricks,  often  several  courses  deep,  and  the 
sides  and  ends  are  almost  invariably  soft,  and  thus  the  per- 
centage of  high-grade,  hard,  uniform-colored  bricks  is  greatly 
reduced,  in  the  face  of  a market  continually  calling  for  the 
better  quality  product. 


Kiln  Sheds. 

We  frequently  find  factories  where  the  kilns  are  not  pro- 
tected by  sheds.  The  bricks  are  set  in  the  open,  and  after  the 
first  bench  is  set  and  cased,  it  is  covered  with  boards  or  often 


with  canvas,  which  is  a very  unsatisfactory  arrangement,  re- 
sulting in  considerable  loss  during  inclement  weather  and 
frequent  delays  in  the  setting  operations. 

The  usual  arrangement  is  a large  shed  with  board  roof 
completely  covering  the  kiln,  with  lower  shed  roofs  on  either 
side  to  cover  the  fire  pits.  The  main  shed  roof  has  a monitor 
in  the  center  to  take  away  the  gases  and  watersmoke  escap- 
ing from  the  top  of  the  kiln.  Unless  the  shed  roof  is  high 
above  the  kiln,  it  is  necessary  to  remove  the  roof  boards  dur- 
ing the  high  fire  stages,  particularly  toward  the  latter  period 
of  the  firing,  when  the  top  courses  reach  the  finishing  tem- 
perature. Such  a shed  is  illustrated  in  Fig.  46. 

Setting. 

It  would  too  greatly  extend  this  chapter  to  describe  and 
illustrate  in  detail  the  various  settings  of  scove  kilns.  Each 


158 


BURNING  CLAY  WARES. 


district  has  some  special  feature  in  some  part  of  the  setting 
which  may  be  an  advantage  with  the  fuel  used  and  the  prod- 
uct burned,  but  which  would  be  of  little  or  no  value  in  other 
districts. 

Figs.  39,  40,  41  and  42  illustrate  common-brick  setting  in 
one  district. 

The  end  bench  is  usually  one  and  one-half  brick.  All  the 
bricks  forming  the  arch  are  headers.  The  first  course  is  set 
single  or  in  pairs,  as  shown,  spaced  and  lined  up  to  a straight 
edge  on  the  arch  side.  This  is  backed  up  by  two  stretchers 
to  carry  the  tie  when  the  step-off  course  is  reached.  These 
stretchers  should  not  be  placed  tight  end  to  end,  but  instead 
should  be  separated  about  2 inches  to  form  vertical  flues,  and 
care  should  be  taken  in  setting  these  bricks  that  the  flues  con- 


nect with  the  spacing  between  some  of  the  header  courses,  in 
order  to  get  fire  to  the  end  wall. 

The  second,  third,  fifth  and  sixth  courses  are  set  headers, 
in  pairs,  and  spaced  as  shown  in  Fig.  40.  The  fourth  and  sev- 
enth courses  are  set  tight.  On  top  of  the  seventh  course  each 
course  is  projected  into  the  arch  space,  which  is  closed  on  the 
twelfth  or  fourteenth  course,  depending  upon  the  width  of  the 
arch.  The  eighth,  ninth,  eleventh  and  twelfth  courses  are  in 
pairs,  and  spaced.  The  tenth  and  thirteenth  courses  are  tight. 
This  setting,  it  will  be  noted,  closes  the  top  of  the  arch  tightly, 
as  shown  in  plan,  Fig.  41,  and  the  flames  cannot  take  a direct 
course  upward  through  the  top  of  the  arch,  but  instead  are 
deflected  to  either  side  and  rise  through  open  checker  work. 
This  method  of  setting  just  above  the  arch  closure  course  is 
an  excellent  one,  but  not  common  practice. 

Where  it  is  desired  to  have  some  draft  upward  through 


BURNING  CLAY  WARES. 


159 


the  top  of  the  arch,  the  closure  is  on  the  fourteenth  course, 
which  is  spaced,  and  the  tight  courses  under  it  not  being  in 
touch  end  to  end,  provide  draft  spaces. 

The  end  bench,  from  the  seventh  course,  is  built  up  ver- 
tically on  the  outside  by  means  of  skin  tied  bricks  (set  at  an 
angle),  stretchers  and  headers,  or  broken  bricks,  as  may  work 
out  best. 

When  the  closure  course  is  set  on  the  end  bench,  the  second 
bench  is  started. 

It  is  considered  best  not  to  have  the  benches  in  whole 
brick  lengths — two,  three  or  four,  as  the  case  may  be — but 
instead  to  make  them  two  and  one-half  bricks,  three  and  one- 
half,  etc. 

This  permits  the  use  of  a single  or  double  stretcher  in  each 
course,  which,  should  there  be  any  tendency  for  the  flame  to 
draw  through  from  one  arch  to  the  next,  serves  as  a baffle  to 
deflect  the  gases  upward.  Experienced  brickmakers  consider 
this  important,  but  it  is  a question.  (Very  satisfactory  re- 
sults have  been  obtained  by  a setting  which  we  will  describe, 
the  purpose  of  which  is  to  provide  flues  connecting  the  sev- 
eral arches.) 

The  arch  facings  are  a duplicate  of  the  end  bench,  and  the 
space  between  them  is  filled  with  stretchers  and  headers  as 
may  bond  to  the  best  advantage,  and  all  spaced  either  singly 
or  in  pairs. 

When  the  second  bench  is  brought  to  the  level  of  the  end 
bench,  the  setters  stand  on  the  second  bench  and  the  setting 
is  carried  to  higher  levels,  usually  in  three-brick  benches, 
which  make  a good  working  width,  and,  when  bonded  in,  form 
a substantial  column. 

From  about  midway  of  the  height  to  the  top,  the  end  bench 
and  sides  are  drawn  in  to  form  a battered  wall.  The  end  and 
sides  out  to  the  face  of  the  setting  are  built  up  first  in  simple 
checker  work,  carefully  bonded,  leaving  a rectangular  space 
in  the  heart  of  the  kiln. 

This  central  space  is  sometimes  filled  with  all  skintled  set- 
ting, as  shown  in  Fig.  42,  the  advantage  of  which  is  rapid 
work.  A skillful  setter  can  toss  five  or  more  bricks  into 
place  and  separate  them  at  the  same  time.  Checker  work, 
Fig.  39,  the  bricks  being  set  singly  in  alternate  courses  of 
headers  and  stretchers,  is  common  practice.  In  such  method 
the  setting  is  built  up  in  blocks  three  brick  square,  and  the 
setter  can  work  from  the  front  in  setting  header  courses  and 


160 


BURNING  CLAY  WARES. 


from  the  side  in  setting  stretchers,  and  thus  does  not  have  to 
bend  his  wrists,  and  can  handle  four  or  five  brick  at  a time, 
as  in  the  skintling  method.  Where  the  bricks  have  a ten- 
dency to  kiln  mark,  or  become  discolored  by  the  flame,  and  it 
is  desired  to  get  a higher  percentage  of  first  quality  facing 
bricks,  the  headers  are  set  in  pairs  two  courses  high,  and 
faced,  alternating  with  double  stretcher  courses  similarly  set, 
or  a single  stretcher  course. 

Figs.  43,  44  and  45  show  a special  setting  mentioned  above. 


i 


=j 

j 


13 


iO 


Figure  43. 


The  arches  are  faced  with  header  courses,  as  in  the  first 
method  of  setting,  as  shown  in  Fig.  44,  set  singly  or  in  pairs. 
In  this  we  find  considerable  difference  in  practice,  some  local- 
ities setting  the  spaced  arch  bricks  in  pairs,  single  courses, 
others  in  pairs,  double  courses,  faced,  others  single  bricks  in 
single  courses,  others  single  bricks  in  double  courses,  faced, 
and  others  a combination  of  single  and  double  courses.  The 
feature  of  the  setting  in  Figs.  43  to  45  is  that  the  interior  of  each 
leg  is  set  solid  with  stretchers  from  arch  to  arch,  but  across 


BURNING  CLAY  WARES. 


161 


the  kiln  each  block  of  stretchers  is  spaced  about  iy2  inches 
from  the  next  block  and  the  setting  is  worked  out  to  connect 
these  flues  with  the  spaces  between  the  header  courses  in  the 


arch  facing,  thus  connecting  each  arch  with  the  adjacent 
arches.  The  setting  above  the  arches  is  such  as  to  give  a 
maximum  quantity  of  face  bricks. 


162 


BURNING  CLAY  WARES. 


In  some  districts  the  two  top  courses  under  the  platting 
are  set  closer  than  the  usual  spacing,  the  purpose  being  to 
check  the  flow  of  the  gases  at  the  point  of  escape  and  give 
greater  opportunity  to  recover  the  heat  from  the  gases.  This 
is  the  purpose  of  the  platting,  and  the  closer  setting  of  the 
top  two  courses  increases  the  effect  of  the  platting. 

In  some  yards  it  is  customary  in  the  later  stages  of  the 
burning  to  cover  the  top  of  the  kiln  with  dirt  or  ashes  to  hold 
back  the  heat,  and  such  covering  results  in  a harder  burned 
product  on  top,  but  the  work  of  putting  on  and  removing  the 
dirt,  and  the  dirt  and  dust  sifting  through  the  bricks  when 


they  are  being  drawn,  offsets  the  value  of  the  harder  product 
and  the  use  of  such  dirt  covering  is  not  common. 

However,  when  a hot  spot  appears  on  top,  indicated  by  ex- 
cessive settling,  such  spot  is  covered  with  dirt  to  check  the 
draft  at  that  point  and  to  drive  the  heat  to  the  surrounding 
cooler  areas. 

An  up-draft  kiln,  illustrated  in  Fig.  46,  is  an  open-top  kiln, 
virtually  a scove  or  clamp  kiln,  inclosed  in  heavy,  permanent 
walls. 

The  advantages  over  a clamp  kiln  are : 

(1)  Saving  in  labor  required  to  scove  and  daub  a clamp 
kiln. 


BURNING  CLAY  WARES. 


163 


(2)  Less  radiation  loss. 

(3)  Permanent  furnaces  and  in  consequence  a better  type. 

(4)  The  platting  bricks  may  be  piled  on  top  of  the  per- 
manent walls,  whereas  in  scored  kilns  they  must  be  tossed  up 
and  down  each  burn. 

(5)  The  heavy  walls  hold  the  set  brick  in  place,  elimi- 
nating the  racking  back  to  batter  the  heads,  and  there  is  no 
danger  of  the  mass  of  brick  spreading  to  a dangerous  degree, 
drawing  outward,  and  sections  falling,  thus  exposing  the  burn- 
ing bricks,  as  occasionally  happens  to  a scoved  kiln. 

There  is,  of  course,  the  cost  of  installation,  but  this  is 
quickly  offset  by  the  saving  in  labor. 

The  necessity  of  removing  the  brick  through  the  doorways 
in  the  ends  of  the  kilns  is  a disadvantage  in  some  situations, 
in  that  it  increases  the  average  distance  the  burned  bricks 
must  be  moved,  and  likewise  increases  the  average  distance 
the  unburned  bricks  must  be  moved  in  getting  them  from  the 
drying  hack  to  the  setting  face. 

The  most  general  arrangement  is  to  place  the  kiln  longi- 
tudinally between  the  dryer  and  the  loading  tracks.  The  av- 
erage distance  the  ware  must  be  moved  within  the  kiln  walls 
is  half  the  length  of  the  kiln,  whereas  the  average  distance 
in  the  scove  kiln  is  half  the  width. 

Where  the  burned  product  is  loaded  into  wagons  which 
can  be  backed  into  the  kiln  to  the  working  face,  the  increased 
distance  is  of  no  consequence.  Where  the  loading  is  into  cars 
or  boats,  portable  gravity  carriers  can  be  used  in  moving  the 
burned  product,  which  practically  eliminates  the  item  of  dis- 
tance cost,  and  the  advantage  of  the  scoved  kiln  is  then  almost 
negligible. 

The  setting  in  an  up-draft  kiln  is  the  same  as  a scoved 
kiln,  except  that  the  outside  is  plumb  and  in  close  touch  with 
the  kiln  wall.  Frequently  the  outside  courses  are  set  2 on  1, 
to  give  greater  flue  space  near  the  walls.  The  setting  varies 
from  3 on  1 (three  bricks  on  edge  in  the  length  of  one  brick) 
to  11  on  5,  depending  on  the  length  and  thickness  of  the 
bricks,  the  usual  setting  being  either  3 on  1 or  8 on  3.  Three 
on  one  is  close  setting,  and  more  open  setting  ranges  through 
8 on  3,  5 on  2,  7 on  3,  11  on  5 and  2 on  1.  The  general  setting 
is  seldom  more  open  than  8 on  3,  and  the  wider  settings  given 
are  only  used  in  parts  of  the  kiln  where  the  burning  has 
shown  that  draft  is  needed  to  get  uniform  results.  The  ad- 


164 


BURNING  CLAY  WARES. 


vantage  of  3 on  1 or  8 on  3 setting  is  that  the  work  can  be 
carried  up  in  three  brick  benches  over  which  the  setters  can 
reach  nicely. 

Sizes  of  common  bricks  vary  widely,  and  the  setting  varies 
accordingly.  The  standard  for  common  bricks  adopted  by  the 
N.  B.  M.  A.  is  8:*4x4x2:‘4.*  The  New  England  and  Hudson 
river  bricks  range  between  7%x3%x2  and  8x3%x2 y8.  The 
Western  and  Southern  product  runs  as  large  as  8%x41/4x2  y2. 

Furnaces  and  Burning. 

Scoved  kilns  were  originally  burned  with  wood  and  the 
furnaces  were  merely  openings  through  the  wall,  connecting 
with  the  arch.  As  wood  became  scarce  and  the  use  of  coal 
was  necessary,  a short,  flat  grate  was  introduced,  and  to  get 
the  heat  to  the  center  an  occasional  stick  of  wood  was  pushed 
into  the  arch  as  near  the  center  as  possible,  or  lumps  of  coal 
were  thrown  in  over  the  grate  fires  to  the  center.  Thus  part 
of  the  combustion  took  place  on  the  grate  bars  and  part  on 
the  solid  kiln  floor. 

The  up-draft  kilns  with  heavy  walls  give  opportunity  for 
permanent  furnaces  of  a better  type.  The  most  common  is  a 
simple  flat  bar  box  furnace  with  a door,  ash  pit  and  throat 
connecting  with  the  kiln  arch,  the  grate  bar  being  level  with 
the  kiln  floor,  and  the  coal  storage  and  firing  pit  depressed 
18  inches  to  24  inches  to  the  level  of  the  ash-pit  floor. 

The  furnaces  are  12  inches  to  16  inches  wide  and  36  inches 
to  48  inches  long,  and  are  spaced  to  correspond  with  the 
arches  of  the  kiln — 36  inches  to  42  inches.  There  is  a differ- 
ence of  opinion  in  regard  to  the.  furnace  throats.  Some  hold 
that  the  throat  should  be  greatly  reduced,  as  small  as  6x6 
inches,  to  introduce  the  combustion  gases  as  a jet,  which 
serves  to  project  them  to  the  center  of  the  kilns.  We  have, 
however,  seen  excellent  results  with  furnace  throats  the  full 
size  of  the  furnace  and  practically  the  size  of  the  arch.  We 
are  of  the  opinion  that  the  experience  and  intelligence  of  the 
burner  has  more  to  do  with  the  results  than  an  open  or  re- 
stricted throat. 

We  frequently  find  a single  furnace  on  each  side  of  the 
kiln,  arranged  for  two  arches,  as  shown  in  Fig.  47,  and  the 


♦Footnote. — Since  the  above  was  written  the  N.  B.  M.  A. 
has  adopted  for  common  brick  the  size,  8x2  *4x3%  inches. 

For  face  brick,  the  size,  8x21/4x3%  inches. 


BURNING  CLAY  WARES. 


165 


flat  bar  coking  table  furnace,  previously  illustrated,  has  been 
extensively  used  in  up-draft  kilns.  Sometimes  three  arches 
are  connected  with  a single  furnace  in  a similar  manner. 

Considerable  skill  in  burning  is  required  to  get  satisfac- 
tory results  from  up-draft  kilns.  It  must  be  borne  in  mind 
that  the  mass  of  set  bricks  is  the  stack,  and  if  any  portion 
becomes  unduly  heated  the  draft  will  be  to  this  point.  Chim- 
neys of  over-burned,  frequently  fused  bricks,  surrounded  by 
masses  of  under-burned  bricks,  are  of  frequent  occurrence, 
and  any  burner  who  can  get  uniform  results  throughout  the 
kiln  should  be  kept  on  the  job.  If  the  kiln  is  wide  and  the 
draft  weak,  the  heads  will  become  unduly  heated  and  the 
center  cannot  be  brought  up  to  a finishing  heat.  The  restricted 


Figure  47. 


throat,  above  mentioned,  is  used  to  overcome  this  difficulty. 
Where  wood  is  used,  crossfiring  is  common  practice.  When 
the  kiln  is  ready  to  fire,  the  fires  are  started  on  one  side 
only,  for  a short  period.  This  serves  to  heat  up  the  brick 
along  this  side,  and  after  this  is  accomplished  the  furnaces 
are  closed  by  sheet  iron  from  plates  and  daubed.  The  firing 
then  is  from  the  other  side  for  a period  of  four  to  six  hours. 
The  strongest  draft  is  on  the  side  first  fired,  and  the  tendency 
is  to  draw  the  gases  through  the  arch  to  this  side,  thus  heat- 
ing the  arch  the  full  width  of  the  kiln,  but  in  spite  of  this 
the  side  next  to  the  fire  rapidly  gains  and  becomes  the  hottest. 
After  about  six  hours  the  fires  are  reversed.  Two  to  four 
days  of  such  cross-firing  gets  off  the  watersmoke  and  heats  up 


166 


BURNING  CLAY  WARES. 


the  kiln  from  side  to  side,  with  the  heads  slightly  in  advance 
of  the  center.  Then  both  sides  are  fired,  some  wood  being 
pushed  to  the  center  to  maintain  the  desired  condition  until 
the  heat  works  to  the  top  and  the  kiln  is  finished.  In  some 


instances  the  cross-firing  is  continued  to  the  end  of  the  burn. 

Cross-firing  is  not  used  in  coal  burning,  and  in  some  op- 
erations a dead  wall  is  set  in  the  center  of  the  arch  to  insure 
independent  control  of  each  furnace.  To  get  independent 
control  of  the  center  and  heads,  we  have  seen  furnaces  con- 


BURNING  CLAY  WARES. 


167 


structed  as  shown  in  Fig.  48.  There  are  two  throats,  one  di- 
rect into  the  arch  and  one  into  a flue  under  the  arch  floor  with 
an  inlet  six  or  more  feet  from  the  kiln  wall.  These  throats 
may  be  plugged  with  loose  bricks  as  the  condition  of  the  kiln 
requires. 

Another  method  of  getting  the  heat  distributed  through- 


Figure  49. 


Figure  50. 


out  the  arch,  in  common  use  in  some  sections,  is  a long 
grate  bar  extending  ten  or  more  feet  into  the  arch  on  each 
side  as  shown  in  plan  in  Fig.  49,  each  grate  of  the  same 
length,  or  alternate  long  and  short  grates,  as  shown  in 
Fig.  50.  A longitudinal  section  of  such  grates  is  shown  in 
Fig.  51.  The  grates  are  straight,  flat  bars,  full  length,  and 
set  on  edge,  or  short  sections  of  cast  gridiron  grates.  The 


168 


BURNING  CLAY  WARES. 


long  bars  are  supported  by  notched  bearing  bars,  as  shown  in 
Fig.  52,  which  span  the  ash  pit  and  are  spaced  about  three 
feet  apart.  The  ash  pits  must  be  kept  clean,  otherwise  the 
bars  and  supports  will  quickly  burn  off.  This  is  done  with  a 
long-handled  swivel  scoop,  as  shown  in  Fig.  53. 

Kilns  have  been  constructed  with  the  furnaces  projecting 
into  the  kiln  as  shown  in  Fig.  54,  and  one  design  has  an  open- 
ing in  the  top  of  the  furnace  arch,  as  shown,  besides  the 
regular  throat  into  the  kiln  arch.  This  type  of  furnace,  with- 


—t  r + 


Figure  51. 

I ■■■■■■■■! 

fill  oar 

Figure  52. 

out  the  top  opening,  found  some  use  in  Pennsylvania  with 
anthracite  coal  for  fuel. 

The  heat  conducted  through  the  furnace  crown  aided  in 
bringing  up  the  heads  and  keeping  them  in  advance  of  the 
center,  which  is  important,  but  the  objection  is  the  bad  setting 
in  consequence  of  the  permanent  non-setting  bench  supporting 
part  of  the  mass  of  bricks. 

A crude  pit  furnace  shown  in  Fig.  55  is  used  in  one  or  two 
installations  and  is  said  to  give  excellent  results,  but  that  it 
should  do  so  is  almost  inconceivable. 

Natural  gas,  oil  and  producer  gas  are  readily  applied  to 


BURNING  CLAY  WARES. 


169 


■up-draft  kiln-firing,  as  one  can  see.  The  pressure  behind  such 
fuel  serves  to  drive  the  gases  to  the  center  and  thus  get  heat 
throughout  the  arch. 

Mr.  J.  D.  Pratt,  in  a paper  before  the  Wisconsin  Clay  Man- 
ufacturers’ Association,  gave  brief  instructions  for  firing  an 
up-draft  kiln,  from  which  we  take  the  following  excerpts : 

“We  use  double  platting,  put  on  in  checker-board  fashion, 
and  raise  every  fourth  brick,  start  wood  fires  in  the  ash  pit 
with  the  ash  pit  wide  open  and  the  furnace  doors  closed,  and 
continue  this  firing  until  the  kiln  has  a good  draft,  then  raise 
fires  onto  the  grates.  Fire  with  smokeless  coal,  with  ash  pits 
and  furnace  doors  wide  open.  After  the  kiln  is  dry,  com- 
mence to  heat  up  with  the  coal  regularly  used  in  burning. 
Increase  the  heat  gradually,  keeping  the  ash  pits  open  and  the 
furnace  doors  closed,  except  about  an  inch  for  air  and  over- 
draft. If  the  center  is  hard  to  get,  set  it  more  open.  After 
the  kiln  is  hot  all  over  the  top,  and  the  platting  down,  com- 


mence the  hot  firing.  It  is  important  to  admit  the  proper 
amount  of  air  and  no  more,  and  this  is  what  the  burner  has 
to  look  after.  Keep  the  grates  and  ash  pits  clean  so  the  fire 
shows  bright  in  the  ash  pits.  Keep  the  furnace  door  open 
three-quarters  of  an  inch  to  one  inch.  Keep  the  grates  cov- 
ered with  not  more  than  three  to  four  inches  of  coal,  and 
fire  light  and  often.  Clinker  with  the  furnace  hot  every  six 
hours,  and  oftner  if  necessary.  Never  open  the  door  to  drive 
the  heat  to  the  center.  Keep  the  furnaces  as  hot  as  the  arches 
will  stand  and  keep  even  fires. 

Summarized,  Mr.  Pratt’s  instructions  are : Use  judgment, 

give  the  operation  careful  attention,  keep  the  fires  in  good 
condition  and  at  their  maximum  efficiency,  and  go  slow.  The 
summary  will  apply  to  any  firing,  but  the  specific  directions 
do  not  apply. 

In  the  Hudson  river  district  it  is  common  practice  to  “fol- 
low up  the  watersmoking” — in  other  words,  do  not  wait  until 
the  steam  is  off  before  raising  the  heat,  but  to  drive  the  heat 


170 


BURNING  CLAY  WARES. 


to  the  maximum  temperature  under  the  steam.  Such  practice 
would  ruin  the  product  in  many  districts. 

One  operation  uses  very  heavy  firing  at  compartively  long 
intervals.  The  effect  is  distillation  of  a large  volume  of  gas 
following  the  firing,  which  is  carried  into  the  kiln  and  burned 
in  contact  with  the  ware  to  whatever  extent  it  is  possible 
to  get  the  necessary  oxygen.  Some  oxygen  will  come  through 
the  furnaces  with  the  gas,  some  from  leakage  through  the 


Figure  54. 


walls,  and  some  from  reduction  of  the  minerals  in  the  clay. 
As  the  fires  burn  down  the  temperature  advances,  then  be- 
comes stationary  as  the  air  excess  increases,  resulting  in  oxi- 
dation. The  result  of  this  reduction,  smoking,  heating,  oxi- 
dizing process,  is  a chocolate  colored  product,  and  the  burns 
are  quite  uniform  and  hard  throughout  the  kiln,  without  an 
excessive  fuel  consumption.  Such  firing,  however,  would  ruin 


BURNING  CLAY  WARES. 


171 


the  arch  bricks  made  from  other  clays,  and  with  the  arches 
down  the  product  above  them  could  not  be  satisfactorily 
burned. 

Firing  is  a problem  of  the  clay  as  well  as  the  fuel,  and 
unless  one  has  specific  knowledge  of  both,  he  cannot  give  de- 
tailed directions  in  regard  to  the  firing.  Having  determined 
the  proper  method  for  a particular  operation,  the  economy  in 
fuel  and  the  character  of  the  results  depend  upon  the  care  and 
intelligence  given  to  the  work. 

It  may  be  mentioned  that  attention  should  be  given  to  the 
entire  kiln  and  not  to  the  furnaces  alone.  Cold  spots  must 
be  looked  for  and  worked  out,  by  regulation  of  the  fires,  by 
the  use  of  wood  or  coal  in  the  arches  under  the  cold  spot,  or 
by  freeing  the  draft  through  the  platting,  or  checking  the 
draft  around  the  spot.  Hot  chimneys,  if  taken  in  time,  may 


Figure  55. 


be  checked  by  covering  the  platting  above  them  with  dirt, 
thus  diverting  the  draft  to  the  surrounding  areas. 

The  speed  of  the  burning  depends  largely  upon  the  safe 
burning  range  of  the  clay.  If  the  range  is  short,  we  must 
take  a longer  time  to  work  the  heat  to  the  top,  whereas  with 
a wide  range  we  may  carry  a higher  temperature  in  the 
arches,  resulting  in  rapid  heat  absorption  throughout  the 
kiln,  since  the  rate  of  such  absorption  varies  with  the  differ- 
ence in  temperature  of  ware  and  gases. 

Coaling. 

Mixing  coal  screenings  with  the  clay  is  a common  practice 
in  several  districts,  and  anthracite  and  coke  are  best  for  this 
purpose.  Bituminous  coal  may  not  be  used,  at  least  not  gen- 


172 


BURNING  CLAY  WARES. 


eraliy,  but  lignite  is  practical.  The  anthracite  and  coke  con- 
tain very  little  volatile  matter,  and  in  consequence  do  not 
cause  bloating.  Bituminous  coal  is  not  only  highly  gaseous, 
but  it  burns  rapidly  and  develops  a high  temperature,  result- 
ing in  fusion  of  the  particles  of  clay  in  contact  with  each 
granule  of  coal  before  the  coal  is  fully  consumed.  The  gas 
generated  by  distillation,  or  combustion,  or  reduction  of  the 
clay  minerals,  becomes  entrapped  in  the  fused  mass,  and  its 
expansion  under  advancing  temperature  forms  blebs,  the  com- 
bined effect  of  which  causes  bloating. 

Lignite  is  used  for  coaling,  but  we  do  not  know  how  gen- 
erally it  may  be  used.  It  is  more  gaseous  than  bituminous 
coal,  and  is  rapid  burning,  but  usually,  being  high  in  ash  and 
moisture,  it  does  not  develop  the  heat  that  we  get  from  bitu- 
minous coal,  and  unless  there  is  fusion  of  the  clay  mass, 
bloating  does  not  result.  Likely,  lignite  could  not  be  used  in 
a clay  which  fuses  at  a low  temperature. 

Sawdust  has  been  extensively  used,  but  more  to  develop 
porosity  than  to  aid  in  the  burning.  It,  too,  is  highly  gaseous, 
but  does  not  cause  bloating. 

In  England,  we  understand,  bricks  are  sometimes  burned 
by  the  coal  mixed  with  the  clay  and  setting  the  bricks  with 
coal  dust.  The  mass  is  cased  up  and  ignited,  and  the  problem 
would  be  to  regulate  the  air  admission  to  insure  a combustion 
rate  sufficient  to  develop  a needed  temperature,  and  not  to 
exceed  such  temperature.  It  applies,  to  burning  bricks,  the 
principle  of  charcoal  burning. 

In  this  country  coaling  is  used  only  as  an  aid,  and  it  is 
supplemented  by  furnace  firing.  The  amount  of  coal  varies  in 
different  districts  between  50  and  100  pounds  per  thousand 
bricks.  In  one  district  the  amount  used  is  66  pounds  per 
thousand  bricks.  Double-coaled  bricks  are  set  around  the 
heads,  over  the  top  and  in  the  casing,  where  green  bricks  are 
used  to  scove  clamp  kilns.  The  term  “double  coaling”  does 
not  mean  twice  the  amount  used  in  single  coaling,  but  simply 
means  a larger  amount,  which  varies  from  several  times  to 
ten  times  the  amount  used  in  single  coaling. 

The  coal  is  usually  added  to  the  clay  in  the  pug  mill  or 
soak  pit,  but  we  have  seen  one  operation  where  the  clay  is 
piled  in  the  clay  shed  to  a certain  depth,  then  covered  with 
the  required  depth  of  coal,  followed  by  a second  lot  of  clay, 
and  the  mass  is  then  cut  down  and  fed  into  the  preparing 
machinery., 


BURNING  CLAY  WARES. 


173 


Machine  Handling  and  Setting. 

A thousand  bricks  will  require  6,000  to  7,000  pounds  of 
clay.  This  must  be  dug,  prepared,  and  manufactured  into 
bricks.  These  bricks  contain  approximately  1,500  pounds  of 
water  which  must  be  driven  off,  requiring  an  average  of  300 
pounds  of  coal.  The  dried  bricks  must  be  set  in  the  kiln, 
followed  by  burning,  which  requires  500  or  more  pounds  of 
coal,  and  finally  the  product  is  removed  from  the  kiln  and 
loaded  for  shipment.  There  are  six  handlings — taking  off 
the  green  bricks  and  placing  them  on  cars,  tossing,  setting, 
drawing  (which  involves  tossing),  loading  on  barrows,  loading 
from  barrows  to  cars.  Two  bricks  at  a time  are  handled  in 
the  first  operation,  two  to  five  in  the  second  and  third  opera- 
tion, four  to  six  in  the  fourth,  fifth  and  sixth  operations. 

Consider  a stiff  mud  factory  with  a kiln  shed  1,000  feet  long 
at  the  end  of  which  is  a dryer  120  feet  long,  and  beyond  this 
an  average  distance  of  80  feet  to  the  machine.  The  bricks 
are  handled  on  cars  in  units  of  400  to  700  and  the  average  dis- 
tance traveled  to  and  into  the  kiln  and  return  will  be  around 
1,500  feet,  including  four  transfers. 

The  burned  bricks  are  handled  in  units  of  100  to  120  an 
average  distance  of  150  feet,  including  return,  or  750  feet  for 
500  bricks,  making  a total  distance  of  2,250  feet  for  each  unit 
of  500  bricks.  For  each  1,000  bricks,  we  handle  three  tons  of 
clay  in  small  units  six  times  and  travel  a distance  of  one-half 
to  three-quarters  of  a mile.  We  also  move  a quarter  of  a ton 
of  coal  and  shovel  it  twice  or  three  times. 

It  is  not  surprising  that  the  profits  are  small  at  the  pre- 
vailing prices.  It  is  surprising  that  we  have  been  so  slow 
in  adapting  mechanical  devices  to  do  the  greater  part  of  this 
work.  A majority  of  yards  today  operate  with  dryer  cars, 
carts  and  wheelbarrows,  by  hand  and  horse  power.  The  work 
is  heavy  and  the  wages  paid  are  low,  compared  with  other  in- 
dustries and  in  consequence  brick  yards  have  difficulty  in  hold- 
ing sufficient  labor  to  do  the  work  in  times  of  prosperity. 

A number  of  larger  yards  use  electric  power  to  move  the 
product  from  the  dryer  to  the  kilns,  and  the  electric  truck  is 
being  tried  out  for  the  movement  of  the  burned  product. 

Mr.  Lemon  Parker,  in  the  Transactions  of  the  N.  B.  M.  A., 
gives  the  following  data  relative  to  handling  a variety  of 
burned  clay  ware : 


174 


BURNING  CLAY  WARES. 


Distance,  ft. 
501 
285 
420 


Barrows. 

ft.  Cost,  per  ton. 


Distance,  ft. 


Electric  Trucks. 


Cost,  per  ton. 


$0,256 

0.129 

0.197 


557 

483 

478 

537 

504 


$0.16 

0.121 

0.145 

0.179 

0.176 


av.  402 


$0,194  average  512 


$0,156 


The  barrow  work  for  512  feet  on  the  same  basis  as  for  402  feet 
will  cost  $0,247  per  ton. 

The  use  of  electric  haulage  will  lessen  the  cost  of  moving 
bricks,  but  does  not  touch  the  several  handling  operations. 
A portable  elevator  has  been  used  to  elevate  the  cars  of  bricks 
in  the  kiln  to  the  setting  level,  thus  reducing  in  some  degree 
the  setting  labor,  but  its  particular  advantage  would  be  in 
higher  setting.  It  would  eliminate  the  tossing  in  so  far  as  a 
setter  could  reach  the  bricks  on  the  car  without  changing  his 
position.  When  the  bricks  on  the  far  end  of  the  care  are  be- 
yond reach,  it  is  better  to  have  a tosser  than  to  require  the 
setter  to  walk  back  and  forth  in  order  to  reach  and  set  the 
bricks.  Furthermore,  a setter  can  place  more  bricks  when 
they  are  tossed  to  him  than  he  could  if  he  had  to  pick  them 
from  the  car.  There  would  be  some  gain  in  labor,  particularly 
if  the  setting  was  of  such  height  as  to  require  an  intermediate 
tosser,  but  considering  the  time  required  to  move  the  eleva- 
tors, etc.,  the  net  gain  would  not  be  large. 

The  Scott  System,  installed  in  a number  of  yards,  extends 
the  take-off  belt  (from  a stiff  mud  machine)  the  full  length 
of  the  kilns,  in  one  instance  over  800  feet.  Opposite  each  kiln 
a cross  conveyer  carries  the  bricks  into  the  kiln.  One  man 
can  transfer  50,000  bricks  per  day  from  the  long  take-off  belt 
to  the  cross  conveyor.  Take  off  and  dryer  transfer  men  are 
thus  eliminated,  except  the  one  man  above  mentioned. 

The  bricks  are  set  in  the  kiln  6 to  8 courses  high  all  over 
the  kiln  floor.  The  conveyor  is  centered  longitudinally  in  the 
kiln  and  within  reach  of  the  conveyer,  the  setters  can  pick  the 
bricks  from  the  belt  and  set  them,  but  when  the  setting  is  be- 
yond reach  a tosser  is  needed.  Whether  there  is  any  gain  in 
the  tossing  and  setting  labor  by  this  method  over  the  ordinary 
method  may  be  questioned,  but  there  is  a decided  gain  in  the 
pace  set  by  the  machine.  The  bricks  have  to  be  set  as  fast  as 
they  are  delivered  to  the  kiln,  or  if  not  they  are  carried 
through  the  kiln  and  dumped  off  from  the  end  of  the  conveyor, 


BURNING  CLAY  WARES. 


175 


at  the  end  of  the  kiln  outside,  where  they  are  mute  evidence 
of  slack  work  or  insufficient  setting  force.  Men,  as  a rule, 
do  not  like  to  be  “snowed  under”  or  “buried,”  as  tl  saying  is, 
and  will  work  harder,  within  a reasonable  limit,  to  have  a 
clean  record  at  the  end  of  the  day,  than  they  will  where  the 
onus  of  a bad  record  may  be  shifted  to  any  slacker  in  the 
crewr. 

The  bricks  so  set  are  dried  at  night  in  the  kiln,  or  two 
nights  and  a day  may  be  used,  if  the  setting  is  in  two  kilns, 
alternately.  The  second  day,  the  conveyer  is  raised  to  permit 
a second  setting  of  6 or  8 courses  on  top  of  the  first  setting, 
and  third,  fourth  and  fifth  settings  follow,  in  the  same  man- 
ner, until  the  kiln  is  filled  to  the  top. 

It  was  intended  to  use  the  same  conveyer  system  in  re- 
moving the  burned  bricks  from  the  kiln,  but  this  does  not 
work  out  well,  as  one  can  readily  see.  The  first  shipments 
from  a kiln  would  be  all  soft  top  bricks,  followed  by  practically 
all  hards,  and  finishing  with  all  arch  bricks.  Such  shipments 
do  not  properly  fill  the  orders. 

The  Fiske  System,  introduced  a number  of  years  ago  in 
New  England  was  a radical  departure  from  established  meth- 
ods. The  bricks  (stiff  mud)  were  racked  in  unites  of  1,500 
on  forms  on  the  factory  floor  at  the  take  off  belt.  An  electric 
traveling  crane  picked  up  the  mass  of  bricks,  by  means  of  a 
lifting  rig  equipped  with  a series  of  fingers  spaced  to  corre- 
spond v7ith  the  setting  on  the  forms.  Each  unit  was  lowered 
by  the  crane  into  a dryer,  then  conveyed  to  the  kiln,  and  set 
in  place  by  the  crane,  and  after  being  burned,  it  was  the  in- 
tention to  similarly  remove  the  bricks  from  the  kiln  and  con- 
vey them  to  the  shipping  station.  This  system  required  an 
open-top  kiln,  or  one  with  a removable  crown,  as  in  the  initial 
installation.  One  crane  man  and  one  helper  sufficed  to  put 
the  bricks  into  the  dryer  and  thence  into  the  kiln.  The  crane 
was  also  used  to  handle  the  dryer  and  kiln  covers  and  bring 
in  the  kiln  coal. 

Following  the  Fiske  initial  installation,  a somewhat  similar 
system  was  developed,  in  the  West,  for  dry  pressed  bricks. 
The  bricks  were  handled  in  smaller  units  and  taken  from  the 
press  to  and  into  the  kiln  on  a single  rail  overhead  trolley 
(telegraph  track)  and  a kiln  crane  was  used  to  place  the 
bricks. 

The  outcome  of  these  systems  is  the  machine  setting,  used 
chiefly  in  Chicago.  The  bricks  are  placed  on  cars  from  the 


176 


BURNING  CLAY  WARES. 


take  off  belts  in  units  of  600  to  1,000  bricks,  and  the  car  setting 
corresponds  to  the  required  kiln  setting — arch  and  bench  units, 
head  units,  casing  and  regular  setting  units.  The  cars  are 
taken  from  the  dryer  to  the  kiln  on  transfer  and  kiln  tracks, 
in  the  usual  manner,  except  that  motor  trams  are  used  for  the 
haulage,  handling  several  cars  at  each  trip.  A traveling  lift- 
ing crane  with  a fingered  lift  picks  up  the  units  from  the  cars 
and  places  them  in  the  kiln  in  the  proper  place. 

A simple  crane  handling  in  use  has  the  usual  car  system, 
delivering  the  cars  of  bricks  to  the  kiln,  where  they  are  picked 
up  by  a crane  and  placed  at  the  setting  face  convenient  to  the 
tossers  and  setters,  the  setting  being  done  by  hand.  Platforms 
with  turn  tables  are  placed  in  the  kiln  at  intervals  convenient 
for  the  setters.  The  cars  of  dried  bricks  are  placed  on  these 
platforms,  and  in  setting  the  bricks,  as  soon  as  one  side  or  end 
of  each  car  is  emptied  the  car  may  be  turned  on  the  table, 
bringing  the  other  side  or  end  within  reach  of  the  setters. 

After  the  bricks  are  burned,  the  kiln  is  emptied  by  piling 
the  bricks  on  platforms  which,  when  loaded,  are  picked  up  by 
the  crane,  carried  to  the  car  and  dumped,  the  platforms  being 
returned  to  the  kiln.  The  tossers,  both  in  setting  and  unloading 
the  kiln,  are  largely  eliminated,  and  the  wheelers,  from  the 
kiln  to  the  car,  are  replaced  by  one  crane  man  and  a car 
trimmer. 

The  up-draft  kiln,  or  any  open-top  kiln,  is  especially  adapted 
to  machine  operations. 

Open-top  Continuous  Kilns. 

The  advantages  of  the  periodic  clamp  and  up-draft  kilns 
have  been  pointed  out — low  first  cost,  sanitary  conditions, 
large  capacity,  economy  in  fuel,  availability  for  storage,  and 
adaptability  to  mechanical  devices  for  handling  the  product. 

The  objections  are  the  commonly  large  percentage  of  soft 
bricks,  the  frequently  inferior  arch  bricks,  and  the  excessive 
loss  in  broken  bricks. 

There  is  an  insistent  demand  for  an  open-top  continuous 
kiln,  particularly  in  the  South.  Such  a kiln  is  comparatively 
low  in  cost ; it  is  as  sanitary  as  the  up-draft  kiln,  which  is  an 
important  feature  in  hot  climates ; the  capacity,  single  fired, 
is  limited  to  from  30,000  to  50,000  bricks  per  day,  but  this  is 
sufficient  for  the  average  plant;  being  regenerative,  the  kiln 
is  more  economical  in  fuel  than  a periodic  kiln ; it  is  adapted 


BURNING  CLAY  WARES. 


177 


to  crane  setting  and  drawing;  it  gives  a larger  percentage  of 
hard  bricks  and  largely  eliminates  arch  bricks. 

Several  years  ago  an  attempt  was  made  in  New  England 
to  operate  a scoved  kiln  as  a continuous  regenerative  kiln. 
There  were  no  arches  across  the  kiln,  but  instead  the  bricks 
were  set  to  form  a series  of  longitudinal  flues  in  the  bottom 
as  in  a crowned  ring  kiln  setting  and  vertical  flues  extended 
from  the  trace  flues  to  the  top  of  the  brick  setting.  These 
vertical  flues  were  spaced  about  42  inches  longitudinally  and 
transversely,  and  were  used  as  feed  holes.  The  kiln  was  cased 
and  platted  in  the  usual  maimer,  except  the  tops  of  the  feed 
holes  were  provided  with  suitable  caps. 

At  intervals  of  approximately  16  feet  corresponding  to  sec- 
tions of  a continuous  kiln,  were  underground  transverse  draft 
flues  connected  with  a lateral  main  (fan)  draft  flue  outside 


m 


Figure  56. 

the  kiln.  The  draft  flues  within  the  kiln  area  had  graduated 
openings  in  the  kiln  floor. 

The  operation  was  identically  the  same  as  that  of  an  open- 
top  continuous  kiln  or  a crowned  ring  kiln. 

It  could  hardly  be  expected  that  such  a radical  departure 
from  the  usual  scove  kiln  operation  would  result  in  a perfect 
burn  in  the  first  attempt,  but  as  far  as  could  be  determined 
from  an  examination  of  the  product  in  both  types  of  kilns  on 
the  same  yard,  the  advantage  was  with  the  continuous  opera- 
tion. The  burn  was  lacking  in  hardness  but  otherwise  was 
better  than  that  in  the  regular  scove  kiln,  and  undoubtedly 
had  the  test  been  repeated,  there  would  have  been  improve- 
ment in  the  results. 


178 


BURNING  CLAY  WARES. 


It  is  but  a step  from  such  a crude  continuous  operation  to 
the  open -top  continuous  kilns  which  are  successfully  operated 
in  Europe,  and  there  are  a few  successful  installations  in  this 
country, 'besides  several  failures.  A kiln  principle  is  not  to  be 
condemned  because  of  initial  failures  to  adapt  it  to  American 
conditions,  otherwise  we  would  not  now  have  in  operation  the 
several  hundred  continuous  kilns  of  various  types.  There 
have  been  a number  of  disappointments  and  several  flat  fail- 
ures in  the  development  of  continuous  kilns,  but  in  spite  of 
these,  the  regenerative  principle  has  won  its  way  and  now  is 
a large  factor  in  the  ceramic  industries. 

The  Chmelewski  (Finland)  kiln,  patented  in  this  country, 


and  offered  to  the  clayworkers  several  years  ago,  is  an  open 
top  continuous  kiln.  It  is  shown  in  Fig.  56  (plan),  Fig.  57 
(cross  section)  and  Fig.  58  (longitudinal  sections).  It  is  sim- 
ply a ring  kiln,  without  a crown. 

In  the  original  kiln,  the  tunnel  is  about  12  feet  wide  and 
10  feet  high,  in  sections  16  to  20  feet  long,  each  section  con- 
trolled by  a single  inlet,  at  the  floor  level,  into  the  main  draft 
flue  between  the  parallel  tunnels.  The  sections  are  closed  by 
sheet  iron  plates  lowered  from  the  top  into  a slot  provided  in 
the  brick  setting.  The  bricks  are  set  to  form  longitudinal 
flues  in  the  bottom  spaced  about  three  feet  apart,  and  each 
flue  is  8 inches  to  9 inches  wide  and  the  height  of  four  (Ameri- 
can) bricks.  Cross  flues,  10  inches  wide  and  16  inches  high, 
and  spaced  five  feet  apart  are  also  set  in  the  bottom.  These 
cross  flues  correspond  with  the  arched  openings  in  the  outside 
kiln  wall.  Vertical  flues  spaced  about  three  feet  apart,  extend 
from  the  top  of  the  longitudinal  flues  to  the  top  of  the  setting. 
These  flues  are  about  six  inches  square  and  serve  as  stacks 


BURNING  CLAY  WARES. 


179 


in  the  early  stages  of  the  drying  and  as  feed  holes  and  fire 
ducts  during  the  burning. 

The  set  bricks  are  covered  with  two  courses  of  platting 
and  these  with  several  inches  of  dirt.  Openings  are  left  in 
the  platting  corresponding  with  the  vertical  flues  and  these 
are  fitted  with  collars  and  caps  over  the  compartments  under 
fire. 

After  a section  is  set  and  platted,  fires  are  started  in  the 
small  arches  in  the  outside  walls  connecting  with  the  cross 


flues.  The  vertical  flues  open  through  the  platting,  giving  the 
necessary  draft,  and  the  drying  can  be  controlled  by  these 
openings.  By  closing  those  on  the  outer  wall  side  and  open- 
ing those  on  the  inner  wall  side,  the  heat  will  be  drawn  com- 
pletely across  the  kiln,  or  to  any  degree  desired,  by  closing 
the  openings  on  the  inner  side  and  opening  those  toward  the 
outer  wall. 

When  the  bricks  are  dry  enough  and  hot  enough  not  to  be 
injured  by  the  combustion  gases,  not  to  soot,  etc.,  the  damper 
between  the  drying  section  and  the  sections  subject  to  com- 
bustion gases  is  drawn,  and  at  the  same  time,  the  independent 


Figure  59. 


fires  are  extinguished  and  the  small  arches  closed.  Three  or 
more  sections  will  be  drying  at  the  same  time,  the  number 
depending  upon  the  time  required  for  this  work,  in  order  to 
keep  pace  with  the  burning. 

The  coal  firing  is  through  the  holes  in  the  platting  and  is 
the  same  as  any  ring  kiln  firing — a small  quantity  of  fuel  in 
each  hole  at  intervals  of  12  to  20  minutes.  The  setting  is  also 
the  same  as  in  a ring  kiln  (Hoffman)  except  the  independent 
drying  flues. 


180 


BURNING  CLAY  WARES. 


The  Otto  Bock  open  top  continuous  kiln  shown  in  Fig.  59 
(cross  section),  differs  from  the  Chmelewski  kiln  in  that  the 
draft  flues  are  on  top  immediately  under  the  platting,  and  in- 
stead of  a single  draft  outlet  for  each  section,  the  outlets  are 
a series  of  openings  about  five  inches  square,  spaced  about 
ten  inches  on  centers. 

The  tunnel  may  be  below  the  ground  level,  as  shown,  or 
on  the  ground  level,  as  drainage  conditions  may  require.  The 
tunnels  are  about  12  feet  wide  and  5 feet  high,  and  the  bricks 
(German  size)  are  set  10  courses  high  (about  4 feet). 

The  kiln  is  fully  continuous,  having  parallel  tunnels  with 
cross  tunnels  connecting  the  ends.  The  usual  layout  has  nomi- 
nally, 16  compartments  each  about  16  feet  long,  but  since 


Figure  60. 


there  are  no  doorways  nor  flues  at  intervals,  which  would  des- 
ignate sections  of  the  kiln,  the  division  into  sections  is  simply 
a matter  of  working  units. 

There  are  the  usual  sections — heating  up,  burning,  cooling, 
drawing  and  setting,  in  connected  continuous  operation.  After 
a section  is  set  it  is  closed  by  a paper  damper,  and  two  such 
sections  are  watersmoking.  The  operation  is  as  follows : 
sections  1 and  2 are  watersmoking ; section  3,  setting ; section 


1 1 : 

OTMMHMe b b B n n e b 


fei  "b  "£ "ePsT rTi eV  id” e"  b b B'b 


1-1-14- 14  l-i  - 


w 


Figure  61. 


4,  drawing,  and  the  air  for  combustion  enters  in  this  section ; 
sections  5,  6,  7,  8,  9,  10  and  11  are  cooling,  and  the  air  as  it 
passes  from  section  4 to  section  11  advances  in  temperature 
to  a red  heat ; sections  12,  13  and  14  are  burning,  and  the 
products  of  combustion  pass  through  sections  15  and  16,  which 
are  heating  up.  The  dirt  platting  from  section  4 is  being  re- 
moved and  placed  on  section  3 as  the  work  of  drawing  and 
setting  progresses. 

The  K.  W.  Klose  kiln  is  a modification  of  and  improve- 
ment on  the  Bock  kiln,  and  is  illustrated  in  Fig.  60  (cross- 
section),  Fig.  61  (longitudinal  section)  and  Fig.  62  (view 


BURNING  CLAY  WARES. 


181 


plan).  The  draft  outlets  are  a series  as  in  the  Bock  kiln, 
except  they  are  larger  in  size,  not  so  closely  spaced,  and 
are  placed  midway  in  the  vertical  kiln  wall.  (Note.- — In  the 
Bock  kiln  we  have  many  small  draft  openings  immediately 
under  the  platting ; the  Klose  kiln  has  fewer  and  larger  draft 
openings  approximately  midway  between  the  platting  and  the 
kiln  floor ; the  Chmelewski  kiln  has  a single  draft  outlet  for 
each  section  located  near  the  end  of  the  section  on  the  kiln 
floor.  With  such  extremes  in  existing  types  of  the  open-top 
kiln,  one  would  conclude  that  the  success  of  the  kiln  depends 
more  upon  skillful  operation  than  on  the  design.)  The  paral- 
lel tunnels  are  connected  at  the  ends  by  flues  instead  of  con- 
tinuing the  tunnels  across  the  ends.  The  central  flue  is  the 
main  draft  flue,  with  a stack  or  fan  at  one  end  of  the  kiln,  and 
connections  to  this  flue  and  the  kiln  outlets  are  by  means  of 


Figure  62. 

portable  goosenecks.  In  the  outer  walls  are  openings  corre- 
sponding to  the  draft  outlets  in  the  inner  wall,  and  these  con- 
nect with  an  underground  advanced  heat  flue  completely  sur- 
rounding the  kiln,  the  continuity  of  which  is  broken  at  each 
end  by  dampers. 

The  bricks  are  set  in  the  usual  manner  with  longitudinal 
(“trace”)  flues  in  the  bottom  corresponding  to  the  openings  in 
the  end  walls,  and  vertical  flues  connect  these  bottom  flues 
with  the  platting  feed  holes. 

The  operation  is  the  same  as  in  the  Bock  kiln,  or  any  direct 
coal-fired  ring  kiln,  the  processes  of  drawing,  setting,  platting, 
cooling,  burning  and  heating  up  being  continuous  and  advanc- 
ing section  by  section. 

The  initial  water-smoking  is  independent  and  is  done  with 
hot  air  from  the  cooling  sections.  The  water-smoking  sections 


182 


BURNING  CLAY  WARES. 


are  separated  from  the  heating  op  and  setting  sections  by  pa- 
per shields  pasted  to  the  bricks.  Cooling  sections  of  the  kiln 
are  connected  with  the  advanced  heating  flue  by  goosenecks, 
and  at  the  same  time  the  draft  outlets  in  the  inner  kiln  wall 
are  connected  by  goosenecks  with  the  main  draft  flue.  The  hot 
air  from  the  cooling  sections  is  drawn  into  the  advanced  heat 
flue  and  is  by-passed  therein  direct  to  the  water-smoking  sec- 
tions ahead  of  the  heating  up  chambers  or  backward  around 
the  drawing  and  setting  sections,  as  may  be  the  shortest  cut, 
and  the  dampers  in  the  heating-up  flue  give  this  control.  The 
goosenecks  connecting  the  water-smoking  sections  with  the 
heating  flue  introduce  the  hot  air  into  these  sections,  and  simi- 
lar connections  with  the  draft  flue  on  the  opposite  side  of  the 
water-smoking  sections  complete  the  circuit  to  the  fan. 


Fig.  62  illustrates  the  advanced  heating  flue  for  water- 
smoking applicable  to  the  kiln  in  question,  and  to  any  contin- 
uous kiln,  with  such  modifications  as  the  kiln  construction  may 
require. 

A chambered  type  of  open  top  continuous  kiln,  in  principle, 
is  shown  in  Fig.  63  (longitudinal  section),  Fig.  64  (cross-sec- 
tion) and  Fig.  65  (plan). 

The  usual  method  of  building  this  kiln  is  to  have  each  row 
of  compartments  in  a separate  battery  with  working  space  be- 
tween. The  main  draft  flue  will  then  be  underground  between 
the  two  batteries  of  kilns.  The  transfer  tracks  from  the  dryer 
will  be  centered  in  this  working  space,  with  stub  tracks  into 
each  compartment. 


BURNING  CLAY  WARES. 


133 


The  burned  ware  is  removed  through  doorways  in  the  op- 
posite ends  of  the  compartments.  The  ware  is  set  in  the  usuai 
checkers  with  feed  holes  corresponding  with  holes  in  the  plat- 
ting, and  is  direct  coal  fired.  The  bricks  may  be  set  38  to  42 


Figure  65. 


courses  high  and  burned  hard  from  top  to  bottom.  One  in- 
stallation of  a somewhat  similar  type  of  kiln  uses  producer 
gas,  but  the  kiln  is  best  adapted  to  direct  coal  firing.  This 
kiln  has  shown  very  satisfactory  results  and  extended  use  of 
the  kiln  is  promising. 

Periodic  Crowned  Updraft  Kilns. 

A crowned  updraft  kiln  is  seldom  used  in  the  ceramic  in- 
dustries except  in  burning  pottery.  We  see  no  reason  for  the 
extensive  and  continued  use  of  the  updraft  kiln  in  the  pottery 
industry,  except  that  the  operation  is  largely  controlled  by 
tradition.  In  the  near  past  to  ancient  times  it  has  been  an  in- 
dustry of  secrecy,  of  formulas  handed  down  from  father  to  son 
for  generations,  of  processes  carefully  guarded,  and  the  up- 
draft kiln,  adopted  in  the  early  periods  of  the  industry,  has 
come  down  to  the  present  time  along  with  the  other  features 
of  the  work.  There  may  be  good  reasons  for  the  continued  use 


184 


BURNING  CLAY  WARES. 


of  this  type  of  kiln  for  this  industry,  but  we  fail  to  appreciate 
the  full  force  of  them. 

The  fires  are  below  the  floor  level  and  a portion  of  the  gas 
passes  through  flues  under  the  kiln  floor  and  rises  through  a 
center  well  hole  in  the  floor,  then  upward  among  the  ware,  and 
escapes  through  a vent  in  the  crown.  Low  bags  inside  the 
kiln,  connecting  with  the  furnace  throats,  direct  a part  of  the 
gas  from  the  furnaces  upward  into  the  kiln  space,  and  from 
the  top  of  the  bag  the  gas  takes  a vertical  and  diagonal  course 
to  the  draft  outlet  in  the  center  of  the  crown.  Thus  we  have 
a volume  of  gas  rising  from  the  center  well  hole  in  the  floor 
and  volumes  from  each  bag.  In  some  instances  there  are  a 
series  of  holes  in  the  floor  introducing  the  gases  into  the  kiln. 

Montgomery  and  Gray,  in  Yol.  XIV.  Trans.  American  Cera- 
mic Society,  present  data  relative  to  the  fuel  consumption  in 
up-draft  and  down-draft  pottery  kilns.  Gray  shows  a fuel 
consumption  of  about  9 tons  of  coal  per  burn  in  four  up-draft 
kilns,  for  temperatures  of  cone  8,  and  Montgomery  gives  the 
same  tonnage  in  a center-stack  down-draft  kiln,  burning  to 
cone  11.  The  kilns  are  practically  the  same  in  cubic  capacity. 

A single  instance  is  not  sufficient  evidence  of  the  advantage 
of  one  type  over  the  other,  nor  do  we  consider  the  small  dif- 
ference in  fuel  as  an  important  item.  The  uniformity  of  burn 
is  the  chief  concern,  and  we  are  of  the  opinion  that  the  down- 
draft  principle  will  make  the  best  showing  in  this  respect. 
Watts,  in  Yol.  Y,  Trans.  American  Ceramic  Society,  makes  the 
statement:  “Any  one  who  has  studied  the  workings  of  an  up- 
draft white  ware  kiln  knows  that  the  outside  ring  is  solid-flat 
ware.  Between  this  and  the  Center  is  a great  area  that  is  too 
soft  for  flat  ware  and  too  hard  for  hollow  ware.  The  ware 
taken  from  this  area  is  never  right.  Why  can  we  not  use  a 
center-stack  down-draft  kiln,  which  will  give  uniform  results, 
and  burn  only  such  ware  in  a kiln  as  can  be  all  burned  at  the 
same  temperature?” 

From  an  engineering  standpoint,  without  reference  to  pot- 
tery ware,  the  down-draft  principle  has  a decided  advantage 
over  the  up-draft. 

The  first  objection  to  the  up-draft  kiln,  it  seems  to  us.  is 
the  cost  of  maintaining  the  under  floor  flues,  subjected,  as 
they  are,  to  high  temperatures  and  carrying  the  load  of  the 
ware.  This  objection  has  had  effect  on  the  development  of 
under  floor  fired  kilns  for  the  common  ware  industry,  and 
many  up-and-down-draft  kilns  have  been  converted  into  down- 


BURNING  CLAY  WARES. 


185 


drafts  on  this  account,  although  a number  of  them  gave  very 
uniform  results. 

The  floor  of  an  up-draft  kiln  or  any  under  floor  fired  type 
must  be  above  the  factory  floor  level,  and  all  the  ware  has  to 
be  carried  up  into  the  kiln  and  brought  down  after  being 
burned,  thus  increasing  the  labor  cost. 

The  radiation  loss  from  any  kiln  is  greatest  from  the 
crown.  It  is  greater  from  the  crown  of  a down-draft  kiln,  be- 
cause the  temperature  of  the  gases  under  the  crown  of  a down- 
draft  is  higher  than  that  of  an  up-draft,  and,  considered  from 


this  standpoint  alone,  the  up-draft  kiln  would  show  economy 
in  fuel,  but  the  purpose  of  a kiln  is,  first,  to  burn  the  ware 
uniformly,  and  fuel  economy  is  of  second  consideration.  It 
must  be  remembered  that  while  the  radiation  from  the  top  of 
a down-draft  kiln  is  greater  than  in  an  up-draft,  the  conduction 
losses  on  the  bottom  are  less.  The  top  of  an  up-draft  kiln  is 
most  distant  from  the  source  of  heat;  the  gases  there  have  a 
minimum  temperature  and  a maximum  radiation  loss.  The 
down-draft  kiln  has  a maximum  temperature  in  the  top  to  off- 


186 


BURNING  CLAY  WARES. 


set  the  radiation  loss.  The  bottom  of  the  kiln,  where  the  tem- 
perature of  the  gases  is  the  least,  has  a minimum  radiation 
(conduction)  loss,  and  besides  has  the  benefit  of  the  ring  of 
furnaces  around  the  base  of  the  kiln.  The  burning  of  open-set 
ware  is  materially  advanced  by  pressure.  We  can  get  hard 
ware  under  weight  in  the  bottom  of  the  kiln  at  a lower  tem- 
perature than  we  can  in  the  top  of  a kiln  where  the  product 
has  no  superincumbent  load.  This,  of  course,  is  of  no  conse- 


Figure  67. 

quence  in  pottery  ware  enclosed  in  saggers,  but  it  is  impor- 
tant in  burning  common  wares  which  are  stacked  up  in  the 
kiln,  one  piece  on  another,  the  load  increasing  from  top  to 
bottom.  Finally,  another  difficulty  of  the  up-draft  kiln  in 
burning  common  ware  is  that  the  proximity  of  the  ware  in 
the  bottom  of  the  kiln  to  the  fires  subjects  it  to  an  intense 
heat  and  to  reducing  conditions.  The  former  intensified  by 
the  overburden  carried  by  the  bottom  ware  results  in  over- 
burned, distorted  ware,  and  the  latter  causes  discoloration. 


BURNING  CLAY  WARES. 


187 


We  can  get  more  uniform  common  ware  burns  in  down- 
draft  kilns  than  in  up-draft,  and  the  latter  type  of  kiln  is  sel- 
dom found  except  in  the  pottery  industries  including  abrasive 
products. 

We  have  seen  one  or  two  fire  brick  up-draft  kiln  installa- 
tions which  were  scarcely  more  than  crowned  “up-draft”  kilns. 
The  crown  replaced  the  platting,  and  the  arches  were  perma- 
nent trenches  stepped  over  with  the  fire  bricks  to  be  burned. 

Fig.  66  (sectional  elevation)  and  Fig.  67  (plan)  illustrate 
an  up-draft  pottery  kiln,  after  Riddle,  Vol.  XIII,  Trans.  Ameri- 
can Ceramic  Society. 

Down-Draft  Periodic  Kiln. 

The  down-draft  kiln  is  the  type  of  kiln  most  widely  used 
and  in  it  are  burned  the  greatest  variety  of  products — com- 
mon bricks,  face  bricks,  fire  bricks,  paving  blocks,  drain  tile 
and  fire-proofing,  salt-glazed  conduits,  sewer  pipe  and  building 
blocks,  stoneware,  terra  cotta  in  some  degree,  and  the  smaller 
lines  of  special  ware. 

A few  years  ago  it  was  a simple  matter  for  an  engineer 
to  select  the  proper  kiln  principle  for  any  industry,  and  his 
chief  problem  was  to  adapt  the  type  selected  to  the  particular 
industry.  Today  his  biggest  problem  is  to  determine  the  type 
of  kiln. 

Machine  setting  demands  an  open  top  kiln,  and  if  the 
open  top  continuous  kiln,  with  its  economy  in  fuel  and  sani- 
tariness, can  be  developed  to  give  the  better  results  required 
by  critical  markets,  the  down-draft  kiln  must  largely  abandon 
the  common  brick  field. 

The  higher  types  of  continuous  kiln  are  steadily  invading 
the  pre-eminent  field  of  the  periodic  kiln.  The  latter,  how- 
ever, will  never  be  abandoned,  but  instead  will  be  further  de- 
veloped and  specialized  to  fill  a very  important  place  in  the 
ceramic  industries. 

Rectangular  Down-Draft  Kilns. 

The  rectangular  kiln  has  a possible  larger  capacity  than 
the  round  kiln  and  permits  a better  yard  arrangement.  Kilns 
have  been  built  150  feet  long,  holding  approximately  300,000 
standard  size  bricks.  The  further  advantages  of  a rectangular 
kiln  are ; — the  uniform  setting  from  end  to  end,  and  wide  door- 
ways permitting  double  tracks  in  consequence  of  which  the 
setting  crew  is  not  delayed  by  switching  cars  in  and  out,  or 
wagons  can  be  backed  in  to  the  working  face. 


188 


BURNING  CLAY  WARES. 


Multiple  Stack  Kilns. 

The  difficulty  in  working  out  a plan  for  a long  kiln  is  in 
getting  an  equal  draft  distribution,  and  this,  in  the  early  de- 
velopment of  large  capacity  kilns,  led  to  the  adoption  of  mul- 
tiple kiln  wall  stacks,  as  illustrated  in  Fig.  68  (cross  section) 
and  Fig.  69  (plan).  The  objection  to  the  wall  stack  is  that 


Figures  68  and  69. 


it  weakens  the  kiln  wall,  and  the  expansion  distorts  the 
stacks,  frequently  partially  closing  them  and  thus  in  a measure 
defeating  the  purpose  of  the  multiple  stack  in  that  if  the 
stacks  are  not  alike  the  intensity  of  the  draft  will  vary.  A 
feature  of  the  wall  stack  is  that  the  stacks  will  become  heated 
up  early  in  the  burning  process,  thus  increasing  the  draft 


BURNING  CLAY  WARES. 


189 


for  the  water-smoking  and  oxidation  when  strong  draft  is 
needed  and  frequently  not  available  in  outside  stacks.  It 
is  also  claimed  that  the  wall  stacks  keep  the  side  walls  hot 
throughout  the  burn  and  that  we  derive  some  benefit  from 
the  heat  in  the  waste  gases.  This  claim  may  be  questioned. 
After  the  gases  have  been  drawn  from  the  kiln  bottom,  the 
chances  are  that  the  temperature  increases  in  their  passage 
through  the  wall  stack  and  instead  of  giving  up  waste  heat 
to  the  kiln  they  draw  heat  away  from  it. 

The  plan  shows  a double  row  of  perforated  floor  bricks  in 
the  longitudinal  center  of  the  kiln,  but  the  floor  can  easily 
be  made  full  perforated  or  perforated  at  intervals  across  the 
floor  as  may  be  desired.  There  is  a difference  of  opinion  in 
regard  to  the  floor  arrangement.  For  hollow  ware  the  floor 
should  be  fully  perforated  with  very  small  slots,  but  for  bricks 
the  semi-solid  floor  is  considered  the  best.  We  found  that 
with  a fully  perforated  floor  that  we  got  better  results  after 
the  under  flues  were  filled  with  sand  except  over  the  center 
flue,  and  this  led  to  the  use  of  the  plan  as  shown  in  Fig.  69. 
Some  engineers  hold  that  it  is  best  to  have  a complete  circu- 
lation of  the  waste  gases  under  the  floor,  and  this  is  our 
opinion,  and  the  plan  shown  can  be  arranged  for  such  com- 
plete circulation. 

The  several  floor  arrangements  above  suggested  have  been 
tried  out  in  actual  practice,  and  each  has  its  advocates. 

The  Laubscher  rectangular  kiln  has  a solid  floor  with  out- 
lets at  intervals,  and  there  is  complete  circulation  under  the 
floor.  The  main  draft  flue  leading  to  the  individual  stacks  is 
under  the  main  kiln  wall.  The  furnaces  are  the  inclined  grate 
bar  type  and  the  hot  coals  and  ashes  rest  upon  the  arch  of 
the  draft  flue,  the  crown  of  which  is  only  four  inches  thick. 
The  effect  of  this  is  to  heat  up  the  draft  flue  almost  at  the 
beginning  of  the  firing,  thus  giving  stronger  draft  in  the  early 
stages  of  the  burning,  as  well  as  throughout  the  burns. 

The  Dennis  kiln  is  a multiple  stack  kiln,  unique  in  that  it 
is,  practically,  two  kilns  in  one. 

Two  distinct  compartments  are  built  side  by  side  and  con- 
nected. The  stacks  are  in  the  wall  between  the  two  compart- 
ments, and  the  furnaces  are  in  the  outside  walls  on  either  side. 

The  Yates  kiln  carries  the  wall  stacks  up  over  the  crown 
from  each  side  to  the  center,  terminating  in  a series  of  stacks. 
Since  the  crown  is  the  part  of  the  kiln  to  become  heated  up 


190 


BURNING  CLAY  WARES. 


first,  it  would  conduct  this  heat  to  the  stack  flues  resting  on 
the  crown  and  thus  develop  a strong  draft  early  in  the  burn- 
ing process,  particularly  in  the  last  stages  of  the  water-smoking 
and  in  the  oxidation  period. 

Rectangular  Kilns  with  Outside  Stacks. 

The  biggest  problem  in  designing  rectangular  kilns  with 
outside  stacks  has  been  to  get  an  equal  draft  in  all  parts  of 
the  kiln. 

Kilns  of  limited  length  have  been  built  with  a central 
longitudinal  flue  in  the  kiln  bottom  with  a cross  flue  leading 


Figure  70. 


to  the  stack  as  shown  in  Fig.  70,  and  one  double  stack  would 
serve  two  kilns.  The  draft  in  such  kilns  is  weak  at  the  ends, 
where  even  with  uniform  draft  we  have  difficulty  in  main- 
taining the  temperature.  Double  cross  flues  as  shown  by  the 
dotted  lines  in  Fig.  70  also  are  used  and  these  divide  the  kiln 
into  three  sections.  The  several  under  floor  outlets  into  the 
main  longitudinal  flue  are,  or  should  be,  adjusted  in  size  to 
give  a theoretically  uniform  draft  from  the  stack  cross  flue 
to  the  most  distant  points  in  the  main  kiln  flue.  It  is,  how- 
ever, impossible  to  adjust  the  sizes  of  these  outlets  to  get  an 
accurate  distribution  of  the  draft,  but  any  proper  adjustment 
helps. 

Kilns  of  these  types  are  doing  good  work  in  the  hands 
of  competent  burners  who  appreciate  the  importance  of  keep- 
ing the  end  fires  in  better  condition  to  offset  the  weaker  draft. 


BURNING  CLAY  WARES, 


191 


Too  frequently,  however,  the  burner  gives  each  furnace  the 
same  treatment  in  firing  and  clinkering  and  expects  the  kiln 
to  overcome  his  shortcomings.  If  it  does  not  the  fault  lies 
with  the  kiln. 

Many  plants  are  laid  out  with  the  stacks  at  the  ends  of 
the  kilns,  or  a large  single  stack,  or  fan,  for  kiln  control 
through  the  ends  of  the  kilns. 

Sometimes  the  kilns  have  a single  flue  through  the  center 
of  the  kiln  longitudinally  and  the  draft  is  all  from  one  end. 
That  such  a plan  is  decidedly  bad  needs  no  argument.  Sim- 
plicity in  kiln  design  is  desirable,  but  such  a plan  sacrifices 


results  for  simplicity.  A diaphragm  in  the  kiln  flue  as  shown 
in  Fig.  71,  serves  to  divert  the  draft  from  the  end  toward, 
or  to,  the  center  and  gives  a center  draft  as  in  the  preceding 
kiln  with  stack  at  the  side. 

A better  end  draft  worked  out  by  Richardson  is  shown  in 
Fig.  72.  The  kiln  has  two  central  flues  with  perforated  floor 
as  shown  in  Fig.  69,  and  these  are  divided  into  three  sections. 
These  sections  on  each  side  are  controlled  by  blind  draft 
flues  extending  beyond  the  kiln  wall  to  the  stack  or  fan  flue, 
and  each  draft  flue  has  a damper  control  just  outside  the  kiln 
wall.  Short  blind  flues  connect  the  kiln  perforated  floor 


192 


BURNING  CLAY  WARES. 


Figures  73  and  74. 


BURNING  CLAY  WARES. 


193 


flues  with  the  draft  flues.  The  kiln  floor  is  solid  except  over 
the  central  kiln  flues,  similar  to  Fig.  69.  This  arrangement 

gives  individual  control  over  six  sections  of  the  kiln  area, 

and  there  is  full  circulation  of  the  kiln  gases  under  the  solid 
floor. 

A balanced  draft  kiln  with  a stack  at  each  end  is  illus- 
trated in  Fig.  73  and  Fig.  74.  The  feature  of  the  kiln  is  the 

two  main  under  floor  draft  flues  in  front  of  the  bag  walls. 

These  connect  at  their  opposite  ends  with  their  respective 
stacks.  Alternating  blind  cross  collecting  flues  extend  across 
the  kiln  from  the  draft  flue,  and  these  connect  with  parallel 
perforated  floor  flues.  The  collecting  flue  outlets  into  the 
draft  flues  are  graduated  in  size  from  the  stacks  to  the  op- 
posite ends  of  the  kilns  to  give  a theoretically  even  draft  from 
end  to  end  of  the  kiln,  in  so  far  as  is  possible.  Uniform 
draft,  however,  is  not  dependent  upon  this  gradation  of  the 
collecting  flue  outlets.  Assume  that,  in  spite  of  the  gradations, 
the  draft  is  strongest  from  a collecting  flue  at  the  end  of 
the  kiln  nearest  its  stack.  The  weakest  draft  then  will  be 
from  the  alternate  collecting  flue  into  the  opposite  draft  flue, 
since  it  is  the  greatest  distance  from  its  stack.  We  have 
then  the  extreme  end  of  the  kiln  controlled  by  the  weakest 
and  strongest  draft  in  adjacent  collecting  flues,  and  the  op- 
posite end  has  a like  control.  As  we  advance  from  one  end 
to  the  other,  the  strong  drafts  on  one  side  are  becoming  weaker, 
and  the  weak  drafts  on  the  opposite  side  becoming  stronger. 
The  aim  of  the  design  is  to  have  the  drafts  equal  throughout 
but  whether  this  is  accomplished  or  not,  the  sums  of  the  draft 
from  adjacent  pairs  of  collecting  flues  are  equal  from  end  to 
end.  As  an  illustration,  represent  the  strong  draft  in  the 
collecting  cross  flue  at  one  end  of  the  kiln  by  10  and  the  weak 
draft  in  the  adjacent  collecting  cross  flue  to  the  opposite  side 
by  2,  then  the  draft  from  the  next  collecting  flue  on  the  strong 
side  may  be  9 and  the  weak  draft  on  the  opposite  side  3.  The 
draft  from  the  next  pair  will  be  8 and  4,  then  7 and  5,  etc.,  the 
sum  in  every  instance  being  12. 

One  might  think  that  the  strong  draft  would  pull  from  the 
weak,  in  other  words,  an  up-draft  through  the  weak,  and  a 
down-draft  through  the  strong.  This,  of  course,  is  impossible, 
because,  no  matter  how  weak  the  draft  from  any  collecting 
flue  may  be  it  has  a big  stack  developing  it  and  the  strong 
draft  flue  will  satisfy  itself  from  the  non-resisting  gases  com- 


194 


BURNING  CLAY  WARES. 


ing  from  the  furnaces.  The  strong  draft  flue  will  get  the 
largest  volume  of  the  furnace  gases,  but  as  it  approaches  its 
limit  capacity  the  weak  draft  flue  will  begin  to  get  its  share. 

The  collecting  flues  are  spaced  36  inches  on  centers,  so 
that  the  maximum  distance  from  one  strong  draft  flue,  if 
there  be  such,  to  the  next  flue  on  the  same  side  is  6 feet 
and  midway  between  is  a weak  draft  flue  getting  some  of  the 
gases. 

The  plan  shown  in  Fig.  74  has  been  modified  in  several 
ways.  The  collecting  flues  may  have  a perforated  floor,  thus 
eliminating  the  adjacent  connected  perforated  floor  flue,  and 
getting  thereby  a simpler  bottom.  The  perforated  floor  collect- 
ing flues  may  be  spaced  18  inches  on  centers,  thus  getting  a 
perforated  floor  for  hollow-ware — fully  perforated  by  lowering 
the  draft  flues  and  extending  the  floor  flues  to  the  bag  wall — 
or  they  may  be  spaced  to  correspond  with  a three-brick  bench, 
which  is  common  practice  in  floors  for  brick  setting. 

Instead  of  the  perforated  floor  flues  across  the  kiln,  we 
may  have  longitudinal  perforated  floor  flues  as  shown  in 
Fig.  69  by  simply  introducing  the  main  draft  flues  with  their 
corresponding  stacks,  in  place  of  the  multiple  wall  stacks. 

Features  in  the  Construction  of  Rectangular  Kilns. 

Prof.  C.  B.  Harrop’s  article  on  “Kiln  Expansion  and  Brac- 
ing,read  before  the  National  Brick  Manufacturers’  Associa- 
tion and  published  in  the  Transactions  and  also  in  pamphlet 
form,  should  be  in  the  files  of  every  clayworker  for  ready 
reference  and  use  in  constructing  kilns,  and  if  his  instruc- 
tions are  followed,  the  bulged  kilns  which  he  illustrates  will 
be  less  in  evidence  in  clayworking  factories. 

In  designing  a kiln  one  should  have  in  mind  that  every- 
thing entering  into  the  construction  will  expand  except  the 
fire  clay  mortar  and  under  burned  fire  bricks,  and  further 
that  no  restraint  short  of  crushing  the  materials  will  pre- 
vent the  expansion.  Provision  for  expansion  should  be  a 
kiln  draftsman’s  slogan.  The  main  walls  of  a continuous 
kiln  are  usually  built  solid,  and  if  we  do  not  provide  for 
expansion  the  end  walls  will  be  crowded  out  and  cracked 
resulting  in  excessive  leakage.  The  best  method  of  reducing 
this  difficulty  is  by  using  a heavy  vertical  joint  in  the  fire 
brick  work.  As  the  bricks  expand  the  mortar  joint  shrinks, 
and  thus  we  control  the  expansion  locally  and  avoid  the  ac- 


. BURNING  CLAY  WARES. 


195 


cumulated  effects,  at  least  for  a long  period  of  operation. 

Division  walls  should  not  be  bonded  into  main  walls,  but 
instead  should  be  recessed  into  the  main  wall  with  a space 
for  expansion  at  the  end  of  the  division  walls.  In  one  instance 
the  end  walls  were  thick  and  battered  and  the  side  walls 
were  recessed  into  the  end  walls. 

Long  flues  and  feather  walls  may  have  an  occasional  lapped 
or  toothed  joint  with  expansion  space  to  take  up  any  expan- 
sion not  controlled  by  the  mortar  joint.  It  is  very  important 
that  the  expansion  of  long  flues  or  walls  connected  with  cross 
feather  walls  be  controlled  locally,  otherwise  the  creeping  of 
the  flue  will  carry  the  feather  walls,  which  must  in  conse- 
quence be  frequently  replaced. 

Rectangular  kiln  crowns  are  built  in  sections  12  feet  to  16 
feet  long,  and  usually  the  sections  are  separated  by  a 2 inch 
to  4 inch  expansion  space.  This  is  absurd.  The  kiln  wall 
has  no  such  joint,  and  the  crown  will  move  with  the  wall,  no 
more,  no  less.  It  never  creeps  on  the  wall.  It  will  crowd  end- 
wise in  the  rise  and  may  even  close  up  the  expansion  joint 
however  large  this  joint  may  be,  but  in  doing  so  it  will  crack 
in  one  or  more  places  between  the  expansion  joints.  The 
large  expansion  joint  is  a provision  to  facilitate  cracking  and 
serves  no  other  purpose. 

The  crown  should  be  in  sections  to  avoid  rebuilding  the 
entire  crown  when  one  section  requires  renewal,  but  there 
should  be  no  space  between  the  sections,  at  least  not  to  ex- 
ceed y2  inch.  We  have  seen  arches  60  feet  long  in  one  section 
and  without  a crack  in  them  after  several  years’  use. 

Where  flues  extend  through  the  main  wall,  such  as  main 
draft  flues,  an  opening  should  be  provided  in  the  wall  and 
arched  over,  but  the  opening  should  not  hug  the  flue  wall  too 
closely.  A heavy  clay  mortar  joint  between  the  walls  of  the 
opening  and  the  flue  walls  will  permit  the  flue  walls  to  move 
should  they  creep  any,  and  such  a joint  will  crush  and  crumble 
away  under  the  expansion  and  movement  of  the  flue  and  thus 
protect  the  main  walls. 

Where  a stack  is  close  to  a kiln  wall  and  connected  with  a 
long  flue  through  the  kiln  we  usually  break  the  flue  under  the 
kiln  wall  and  use  a stub  extension  of  the  flue  from  the  break 
to  the  stack  to  safeguard  the  stack.  The  kiln  flue  may  enter 
the  kiln  wall  opening  and  be  broken  off,  then  the  stub  flue  ex- 
tends from  inside  the  kiln  wall  opening  to  the  stack  opening. 


108 


BURNING  CLAY  WARES. 


Half  circle  arches  on  rectangular  kilns  are  not  satisfactory. 
They  have  a marked  tendency  to  bulge  on  the  quarters  and 
flatten  in  the  center.  The  crown  should  be  the  segment  of  a 
circle  arch  with  a radius  that  will  give  a rise  between  % and 
y3  of  the  width  of  the  kiln.  The  best  crown  construction  is 
built  of  wedge  bricks  to  properly  turn  the  circle,  but  such 
bricks  have  to  be  made  especially  for  the  work  and  are  seldom 
available.  Wedge  bricks  alternating  with  standards  are 
satisfactory,  but  crowns  should  not  be  built  of  all  standards 
where  a heavy  mortar  joint  is  required  to  keep  the  bed  joint 
radial. 

Prof.  Harrop,  in  his  kiln  expansion  paper  previously  cited, 
calls  attention  to  crown  skew  backs  constructed  of  cut  skews 
and  built  up  with  wedge  brick,  and  shows  that  the  arch  thrust 
is  greater  with  the  latter  skew  than  with  the  former,  and  this 
is  the  only  point  in  his  article  wherein  we  do  not  heartily  agree. 

The  rise  of  the  arch  is  a factor  in  the  determination  of  the 
thrust,  and  using  the  same  rise  for  the  crown  circle,  he  has 
a flatter  arch  for  the  wedge  skew  than  for  the  cut  skew, 
and  consequently  gets  a larger  thrust.  If  we  make  the  total 
kiln  height  the  same  in  both  cases,  and  lower  the  spring  line 
to  compensate  for  the  higher  wedge  skew  instead  of  flattening 
the  crown  circle,  we  will  get  practically  the  same  thrust  on 
the  wedge  skew  distributed  over  a larger  area.  The  tendency 
in  a wedge  skew  is  a turning  movement,  while  that  in  a cut 
skew  is  to  slip  horizontally.  In  our  experience,  we  have  seen 
cut  skews  forced  out  on  the  wall  in  many  instances,  but  never 
a wedge  skew. 

Sand  Pockets. 

The  under  floor  flue  system  should  be  arranged  to  provide 
for  an  accumulation  of  sand,  thus  keeping  the  draft  flues  and 
outlets  to  their  maximum  efficiency,  or  provision  should  be 
made  for  easy  cleaning.  We  occasionally  lose  a kiln  of  ware 
and  the  excuse  is  that  the  bottom  is  choked  up  with  sand. 
In  some  yards  the  loss  of  a kiln  is  the  signal  that  the  bottom 
needs  cleaning  and  it  is  baxl  practice.  The  perforated  floor 
flues  in  every  case  should  be  deeper  than  their  outlets  into  the 
draft  flues,  and  the  depth  of  the  sand  accumulation  should 
be  frequently  measured  to  insure  timely  cleaning  and  prevent 
the  accumulation  from  getting  over  into  the  covered  draft  flues, 
where  cleaning  is  impossible  except  to  tear  up  the  entire  floor. 


BURNING  CLAY  WARES. 


197 


Kilns  with  central  longitudinal  perforated  floor  flues  as 
shown  in  Fig.  69  and  Fig.  62  are  easily  cleaned.  In  one  fac- 
tory, the  slotted  floor  bricks  were  loose  and  were  taken  up 
after  each  burn  and  reset  in  setting  the  bricks.  If  these  flues 
are  built  deeper  than  the  openings  into  the  draft  flues  as 
shown  in  Fig.  68,  no  cleaning  will  be  required  for  a period  of 
several  months  to  a year. 

Fully  perforated  floors  are  not  so  easily  cleaned,  and  some 
types  are  particularly  difficult.  The  latter  should  have  ample 
pockets  for  a long  period  of  operation. 

In  some  types,  as  Fig.  70,  with  arches  continuing  the  feather 
walls  over  the  draft  flue  to  support  the  slotted  floor  bricks, 
the  flue  may  be  made  large  enough  to  permit  a workman  to 
enter  it  from  the  end,  and  the  cross  flues  sloped  so  the  sand 
will  flow  down  into  the  central  flue  as  shown  in  Fig.  75.  We 


consider  such  a method  a last  resort  because  the  large  flues, 
particularly  the  arches,  are  liable  to  get  out  of  shape  and  the 
high  feather  walls  to  draw  over,  and  in  consequence  repairs 
to  the  floor  are  as  frequent  as  cleaning  the  flues. 

Setting. 

The  setting  in  a rectangular  kiln  is  very  simple.  If  the 
bags  are  straight  flash  walls,  or  segments  of  large  circles, 
the  setting  is  uniform  from  end  to  end  of  the  kiln.  Square 
or  half  circle  bags  require  that  the  space  between  the  bags 
be  filled,  but  this  independent  of  the  main  setting. 

Kilns  with  solid  floors  and  limited  perforated  floor  space 
must  be  set  in  such  a way  as  not  to  restrict  the  floor  outlets, 
and  also  that  the  spaces  in  the  bottom  courses  will  serve  as 
flues  leading  direct  to  the  floor  slots.  For  example,  the  setting 
on  the  floor  shown  in  Fig.  69  should  start  with  stretcher 


198 


BURNING  CLAY  WARES, 


courses  on  the  floor  and  they  should  be  so  spaced  as  to  rest 
squarely  on  the  slotted  floor  bricks  in  the  center  of  the  kiln. 

The  setting  for  Fig.  74  would  start  with  headers  spaced  to 
suit  the  slotted  floor  bricks  and  preferably  a single  course. 


Figure  76. 


Fig.  76  shows  face  brick  setting  and  incidentally  a good 
burn.  The  lower  part  of  the  kiln  is  set  with  roman  bricks 
(114x4x12)  and  the  upper  part  with  standards.  The  latter 
are  set  8 on  3 and  care  has  been  taken  that  each  column, 
three  brick  square,  is  independent.  The  bricks  are  perfectly 


BURNING  CLAY  WARES. 


199 


faced  as  the  illustration  shows.  It  also  shows  the  racking 
back  from  the  top  of  the  bags  and  the  flash  brick  set  on  the 
step-backs  to  protect  the  bricks  from  the  flame  and  to  force 
the  gases  into  the  crown  space. 

Brick  setting  in  down-draft  kilns  varies  from  24  courses 
high  to  32  courses,  and  the  height  of  the  kiln  for  such  ware 
should  not  exceed  13  feet  in  the  center.  For  setting  28  courses 
high,  as  shown  in  the  illustration,  the  crown  space  in  the 
center  is  about  3 feet  high  before  the  bricks  are  burned  and 
on  either  side  the  bricks  are  about  12  inches  from  the  crown. 
If  the  setting  is  lower  than  this  the  kiln  should  be  correspond- 
ingly lower  and  vice  versa. 

Hollow  ware,  including  sewer  pipe,  is  set  higher  than 
bricks  and  the  kilns  for  such  are  built  15  to  18  feet  high  in 
the  center. 

Round  Down-Draft  Kilns. 

The  round  down-draft  kiln  is  more  widely  used  than  any 
other  type,  and  not  without  reason. 

The  furnaces  are  uniformly  spaced  around  the  outer  peri- 
phery of  the  kiln,  and  each  controls  a sector  of  the  kiln  area. 
The  idea  is  that  the  greatest  mass  of  the  ware  is  nearest  the 
source  of  heat,  and,  as  the  distance  from  the  latter  increases, 
the  mass  to  be  burned  decreases  to  the  vanishing  point  in  the 
center  of  the  kiln.  In  practice,  however,  the  operation  does 
not  carry  out  the  sector  idea.  The  hot  gases  rise  from  the 
bags  around  the  inner  wall  of  the  kiln  into  the  space  between 
the  ware  and  the  crown,  then  pass  downward  through  the 
ware. 

Other  reasons  for  the  wide  use  of  the  circular  kiln  are : 

(1)  Lower  initial  cost  per  ton  capacity. 

(2)  Simple  and  efficient  banding. 

(3)  Less  mass  in  the  kiln  construction  relative  to  the 
capacity  tonnage  and  also  proportionately  less  radiating  sur- 
face, thus  reducing  the  heat  losses. 

(4)  Greater  substantiality  and  in  consequence  less  upkeep 
cost. 

(5)  Advantages  in  setting  several  lines  of  wares. 

A circle  incloses  a greater  area  than  that  of  any  other  form 
having  an  outline  equal  to  that  of  a circle,  and  it  follows  that 
the  mass  of  brickwork  in  the  kiln  and  the  wall  surface  rela- 
tive to  the  area  will  be  a minimum  in  circular  kilns.  A single 
band  encircling  the  kiln  wall  suffices  to  hold  the  kiln  together, 
although  additional  bands  are  desirable,  and  it  is  evident 


200 


BURNING  CLAY  WARES. 


to  any  one  that  a spherical  or  spheroidal  crown  is  less  liable 
to  distortion  than  a circular  segment. 

The  advantages  in  setting  undoubtedly  are  frequently  im- 
portant factors  in  the  selection  of  the  round  kiln.  Sewer  pipes, 
for  instance,  are  set  in  a circle  because  the  large  sizes  must 
be  handled  with  a crane,  and  preferably  should  be  equi-distant 
from  the  furnaces.  The  spaces  between  the  bags  (“Pockets”) 
are  filled  with  smaller  sizes — 4-inch  to  10-inch,  preferably 
6-inch  to  8-inch  sizes,  which  have  less  tendency  to  topple  over 
than  the  4-inch  and  which  will  stand  more  severe  burning 
treatment  than  the  10-inch.  Inside  this  circle  will  come  two 
rings  of  6-inch  to  10-inch  sizes  as  a shield  for  the  larger  sizes. 
Then  follow  the  large  pipes  in  one  or  two  rings,  finishing  the 
center  with  such  sizes  as  can  be  set  by  hand. 

Drain  tile  are  similarly  set,  especially  where  large  sizes  are 
manufactured,  but  smaller  sizes  are  frequently  set  in  parallel 
benches,  as  in  rectangular  kilns. 

Bricks  are  often  set  in  circles,  although  this  is  less  con- 
venient than  in  the  straight  benches.  The  advantages  of  the 
circular  setting  is  that,  if  the  benches  of  bricks  are  liable  to 
roll,  as  frequently  happens  with  high-shrinkage  clays  and 
those  having  short  burning  range,  they  cannot  carry  other 
benches  with  them,  because  circular  setting  distributes  the 
tendency  of  the  movement  radially  in  all  directions  and 
checks  it. 

Pottery  kilns  are  built  in  circular  form — in  the  up-draft 
type  to  get  better  distribution  of  heat  around  the  ware,  as  well 
as  up  through  the  center,  and  also  for  uniform  burning  of  two 
or  more  kinds  of  ware,  since  each  ring  is  subjected  to  the 
same  heat  conditions ; in  the  down-draft  because  of  the  center 
stack,  which  is  the  most  popular  type  of  down-draft  pottery 
kiln. 

Muffle  kilns  are  preferably  circular  because  of  the  greater 
substantiality  of  the  crowns,  and  the  center-stack  type,  which 
necessitates  a circular  form,  is  widely  used. 

The  problem  of  uniform  draft  conditions  is  more  easily 
solved  for  the  circular  kiln  bottom  than  the  rectangular. 

We  have  called  attention  to  the  advantage  of  a rectangular 
kiln  for  car  setting,  namely,  that  two  tracks  may  be  laid  in 
the  kiln,  thus  keeping  a loaded  car  at  the  working  face,  where- 
as, in  the  round  kiln  the  empty  car  must  be  switched  out 
before  a loaded  one  may  be  brought  in,  and  the  setters  are 
idle  during  this  switching  period.  If  the  switching  takes  one 


BURNING  CLAY  WARES. 


201 


minute  for  each  car  the  setters  will  be  idle  one  hour  per  day 
in  setting  30,000  bricks.  This  can  be  partially  overcome  if  the 
yard  arrangement  is  such  that  the  cars  may  enter  from  one 
side  of  the  kiln  and  leave  from  the  other.  There  will  be  no 
interruption  in  the  work  except  in  filling  the  center  space.  A 
small  advantage  of  this  arrangement  is  that  the  cars  are  paral- 
lel with  the  setting  face  instead  of  end  on. 

Another  disadvantage  of  the  round  kiln  for  car  operation 
may  be  mentioned.  In  a rectangular  kiln  the  car  of  ware  is 
always  at  the  setting  face  and  alternately  on  each  side  of  the 
center  of  the  kiln,  thus  being  in.  very  close  touch  with  the 
work,  while  in  a round  kiln  the  maximum  distance  of  the  car 
from  the  setting  face  is  half  the  kiln  diameter,  and  whether 
the  ware  is  tossed  or  carried  the  labor  cost  is  increased. 

Types  of  Round  Kilns. 

We  frequently  distinguish  kiln  types  by  the  stacks,  as  cen- 
ter stack,  multiple  wall  stacks,  multiple  stacks  outside  the 
walls,  and  quite  as  often  designate  the  type  by  the  kiln  bot- 
tom, as  center  well,  radial  flues,  ring  flue,  cross-head  flue,  etc. 
The  “and-so-forth”  includes  many  kilns  with  distinct  flue  ar- 
rangement— a number  of  them  good,  but  which  have  not  found 
wide  enough  use  to  give  them  a name. 

We  will  not  attempt  to  follow  the  development  of  the  round 
kiln,  but  instead  will  simply  present  a few  of  the  better  known 
types,  without  taking  up  the  numerous  modifications  which 
have  been  designed  and  patented  as  improvements  upon  the 
original,  except  such  modification  has  features,  in  our  opinion, 
worthy  of  mention. 

General  Construction  of  Kiln  Bottoms. 

The  kiln  bottom  in  general  has  three  sets  of  flues : 

. (1)  Perforated  floor  flues. 

(2)  Collecting  flues. 

(3)  Main  draft  flues. 

The  collecting  flues  may  be  on  the  same  level  as  the  per- 
forated floor  flues,  with  connecting  openings  in  the  division 
walls,  or  they  may  be  under  the  floor  flues,  with  connecting 
openings  in  the  division  diaphragm.  Some  kiln  bottoms  have 
the  flues  all  on  the  same  level,  being  merely  a system  of  open 
work  over  the  entire  bottom. 

Center  Stack  Kilns. 

The  center  stack  type  of  kiln,  illustrated  in  Fig.  77  and 
Fig.  78,  is  widely  used  in  small  brick  and  tile  plants,  and  .with 


202 


BURNING  CLAY  WARES. 


modifications  in  the  floor,  is  recommended  for  pottery.  It  also 
is  extensively  used  in  muffle  kiln  construction. 

The  floor  shown  in  the  illustrations  is  semi-perforated  and 
especially  adapted  for  bricks.  The  perforated  floor  flues  are 
adjacent  to  and  parallel  with  the  collecting  flues  and  the  latter 
have  outlets  through  their  floors  into  the  under  radial  flues 


Figure  78. 


leading  to  the  stack.  The  floor  and  collecting  flues  should  be 
deep  enough  so  .that  the  connecting  openings  in  the  division 
walls  (not  shown  in  the  illustrations)  need  not  extend  to  the 
bottom  of  the  flues.  This  forms  a pocket  for  sand  in  the  bot- 
tom of  the  perforated  floor  flues,  and  if  the  slotted  floor  blocks 
are  taken  up  occasionally  and  the  pockets  cleaned  out,  no  sand 


BURNING  CLAW  WARES. 


203 


can  get  over  into  the  parallel  collecting  flues  nor  into  under- 
lying radial  draft  flues.  If  the  slotted  floor  blocks  are  set 
loosely  they  can  be  easily  removed  and  replaced,  and  it  is  a 
small  matter  to  keep  the  flues  clean,  or  if  the  radial  flues  are 
above  the  outside  ground  level  they  may  be  extended  through 
the  kiln  wall  and  cleaned  from  the  outside  at  any  time  with- 
out disturbing  the  kiln  floor. 

For  tile  or  other  hollow  ware  the  floor  should  be  fully  per- 
forated, which  requires  that  the  collecting  flues  should  have 
slotted  floor  blocks  as  well  as  the  perforated  floor  flues.  For 
this  type  of  floor  the  floor  flues  may  be  a series  of  concentric 
circles.  The  fully  perforated  floor  eliminates  the  sand  pockets, 
but  by  making  the  flues  deep  and  protecting  the  draft  outlets 
by  bridge  walls  on  each  side  of  each  outlet  we  get  sand  pockets 
between  the  outlets.  Then  if  we  use  solid  instead  of  slotted 


blocks  in  the  floor  immediately  over  the  draft  outlets,  the 
sand  cannot  get  into  the  under  radial  draft  flues. 

The  advantages  of  a center  stack  are : 

(1)  Low  stack  and  low  construction  cost. 

(2)  Strong  draft  early  in  the  burning. 

(3)  Uniform  draft  conditions. 

The  objections  are : 

(1)  Kiln  space  taken  up  by  the  stack. 

(2)  Interference  of  the  stack  with  the  setting. 

The  first  objection  is  of  small  moment.  In  some  wares  we 
have  difficulty  in  getting  the  center,  and  the  stack  occupies 
this  space,  thus  shortening  the  time  required  to  bum,  or  in 
the  same  time  we  get  a more  uniform  burn. 

The  second  objection  is  serious  in  many  operations,  espe- 
cially where  cars  are  used  to  bring  in  and  remove  the  ware. 
It  is  of  no  consequence,  however,  for  ware  carried  by  hand  or 
on  trucks  or  barrows. 

A simple  center  stack  kiln  for  common  bricks  is  shown  in 


204 


BURNING  CLAY  WARES. 


Fig.  79.  The  kiln  has  a solid  floor,  upon  which  is  built  the 
center  stack,  with  openings  in  its  base.  The  bricks  are  set 
with  flues  corresponding  with  the  stack  openings.  In  this  way 
what  ordinarily  constitutes  the  kiln  bottom  becomes  a salable 
product,  and  the  fuel  expended  in  heating  up  the  floor  is  ap- 
plicable to  the  ware,  thus  reducing  the  cost  per  ton ; in  other 
words,  it  increases  the  capacity  of  the  kiln  without  increasing 
the  size  or  the  amount  of  fuel  and  labor  required  in  the 
burning. 


Figure  80. 

Multiple  Stack  Kilns. 

The  Eudaly  kiln  has  been  more  widely  used  than  any  other 
individual  kiln,  perhaps,  because  it  was  introduced  at  a time 
when  there  were  fewer  types  from  which  to  choose,  but  it  had 
merit  and  was  a distinct  advance  over  the  average  kiln  in  use 
at  that  time.  It  was  built  round  and  rectangular,  and  its 
chief  competitor  in  round  kilns  was  the  center-well  kiln,  largely 
used  in  sewer  pipe  manufacture.  The  Eudaly  kiln  is  illus- 
trated in  the  lower  half  of  Fig.  80. 

The  bottom  is  divided  into  sectors  corresponding  to  the  fur- 


BURNING  CLAY  WARES. 


205 


naces,  and  each  sector  is  controlled  by  an  individual  wall 
stack.  The  central  ring  of  the  kiln  is  also  controlled  by  a sep- 
arate wall  stack,  connected  with  the  ring  by  a blind  flue.  The 
feather  walls  are  circular  and  are  stepped  over  the  radial  cen- 
ters of  the  sectors  to  form  flues  to  the  several  stacks,  and  all 
being  on  the  same  level,  the  bottom  is  shallow  compared  with 
that  of  a center  well  type  or  any  type  with  a double  set  of 


Figure  82. 


flues.  The  floor  is  fully  perforated,  or  was  in  the  earlier  in- 
stallations, which  was  considered  essential  for  proper  heat 
distribution. 

There  have  been  many  modifications  of  the  Eudaly  kiln,  to 
get  a more  substantial  bottom,  or  a semi-perforated  or  solid 
floor,  but  the  credit  for  the  wide  use  of  this  type  of  kiln  be- 
longs to  Mr.  Eudaly. 


206 


BURNING  CLAY  WARES. 


The  upper  half  of  Fig.  80  illustrates  the  change  from  a 
Eudaly  floor  to  a semi-solid  floor.  If  the  kiln  was  small  in 
diameter,  only  the  radial  flues  were  needed  for  the  perforated 
floor,  but  where  the  diameter  was  large,  lateral  flues  were  ex- 
tended from  the  radial  flues,  forming  a broken  ring  flue. 

A multiple  stack  kiln  of  a distinct  type  is  shown  in  Fig.  81 
and  Fig.  82.  There  are  four  stacks,  which  may  be  in  the  wall 
or  outside,  and  from  each  of  these  a branching  draft  flue  con- 
trols one-quarter  of  the  kiln.  Above  the  draft  flues  are  the 
perforated  floor  and  collecting  flues  for  brick  setting,  but,  as 
previously  noted,  this  type  of  floor  may  be  readily  changed  to 
a fully  perforated  floor. 

The  radial  branches  of  the  draft  flues  extend  nearly  to  the 
center  of  the  kiln  and  control  the  center  and  intermediate 
area,  while  the  lateral  branches  cover  the  circumferential  ring. 
A ring  flue  could  be  used  instead  of  the  angled  branches,  but 
it  would  require  a right  turn,  whereas  since  the  angled 
branches  have  an  obtuse  turn  the  draft  in  the  three  branches 
will  be  more  nearly  equal  in  consequence.  Perfect  adjust- 


ment of  the  kiln  is  possible  by  changing  the  sizes  of  the  several 
openings  from  the  collecting  flues  into  the  draft  flues. 

The  Hook  down-draft  kiln  bottom,  illustrated  in  a view 
plan,  with  the  perforated  floor  removed,  in  Fig.  83,  is  unique 
in  its  arrangement  and  in  the  practical  uniformity  of  the  draft 
in  such  a simple  plan.  The  illustration  shows  the  entire  flue 
system,  and  the  flues  shown  are  covered  with  perforated  floor 
blocks.  The  feather  walls  are  checker  work  except  the  top 
course.  The  circular  feather  walls  in  front  of  the  stack  serve 
as  baffles  where  the  draft  would  be  the  strongest,  and  by  pro- 
portioning the  checker  work  and  varying  the  openings  in  the 
top  floor,  a uniform  draft  is  claimed  for  the  entire  floor  area. 

Single  Outside  Stack  Kilns. 

The  center  well  type  of  outside  stack  kilns,  Fig.  84  and 
Fig.  85,  should  be  mentioned  first  because  of  its  continued  ex- 
tensive use,  particularly  in  the  sewer  pipe  industry. 

The  arrangement  of  the  kiln  may  be  the  same  as  that  of 


BURNING  CLAY  WARES. 


207 


the  center  stack  kiln,  the  difference  being  that  the  waste  gases 
are  drawn  off  from  below  instead  of  from  above  the  well  inlets. 
Since  there  is  no  inside  stack  to  be  considered,  the  well  can 
be  made  much  larger  and  the  inlets  correspondingly  larger  or  in 
greater  number. 

The  principle  of  the  kiln  is  that  the  draft  should  be  strong- 


est in  the  center  because  of  the  greater  distance  from  the 
furnaces. 

It  is  remarkable  what  a difference  of  opinion  there  is  in 
regard  to  the  position  of  the  inlets  into  the  draft  flues.  The 
center  stack  and  center  well  types  have  proven  by  their  ex- 


Figure  86. 


208 


BURNING  CLAY  WARES. 


tensive  use  the  value  of  strong  central  draft.  On  the  other 
hand  a kiln  largely  used  in  the  west  has  the  draft  openings 
around  the  kiln  wall.  Here  we  have  two  extremes  in  success- 
ful operation,  and  from  the  results  obtained  it  would  be  diffi- 
cult to  determine  which  type  is  the  best. 

The  cross  head  flue  type  of  bottom  comes  second,  perhaps 
first,  in  view  of  its  rapid  development  and  the  firm  place  it 
has  in  the  clay  industry.  It  has  not  displaced  the  center  well 
type,  but  it  has  encroached  largely  upon  the  particular  field  of 
the  latter,  and  in  other  lines  it  by  far  has  the  preference.  Its 
simplicity  and  efficiency  appeal  to  the  clayworker.  This  type 
is  illustrated  in  Fig.  86  and  Fig.  87. 

As  the  name  indicates,  there  is  a main,  arched  draft  or  col- 


lecting flue  across  the  kiln  and  from  the  center  of  this  and 
usually  at  right  angles  to  it,  is  the  main  draft  flue  to  the  stack 
or  fan.  On  either  side  of  and  over  the  cross  flue  are  collecting 
and  perforated  floor  flues.  The  former  enter  the  cross  flue  by 
openings  in  the  side  wall  of  the  cross  flue,  and  the  latter 
connect  with  the  former  by  openings  in  division  walls.  Vari- 
ous plans  are  worked  out  to  overcome  any  lack  of  uniformity 
in  the  draft.  In  the  plan  shown  the  distribution  over  the  kiln 
area  is  regulated  by  the  position  of  the  division  wall  openings 
connecting  the  collecting  and  perforated  floor  flues.  The 
longest  collecting  flues,  in  the  central  axis  of  the  kiln  and  con- 
trolling the  largest  floor  area,  enter  the  crosshead  flue  nearest 
the  stack  flue  connection  and  in  consequence  have  the  strongest 
draft,  which  seems  essential  since  they  must  move  the  largest 


BURNING  CLAY  WARES. 


209 


volume  of  gases.  As  tlie  distance  towards  the  ends  of  the 
cross  flue  increases,  and  the  draft  intensity  presumably  weak- 
ens, the  area  drained  by  the  collecting  flues  lessens,  and  a 
practical  balance  is  maintained. 

Some  builders  claim  that  since  the  intensity  of  the  draft 
is  greatest  in  the  center  of  the  cross  flue,  the  inlets  from  the 
central  collecting  flues  should  be  of  minimum  size,  increasing 
toward  the  ends  of  the  cross  flue.  Others  hold  that  the  inlets 
into  the  cross  flue  should  correspond  to  the  floor  area  drained, 
which  would  make  central  connections  large  and  the  end  con- 
nections small.  Both  plans  are  followed  with  good  results. 
The  kind  of  ware  and  setting  has  a great  influence  on  the 


Figure  88. 


draft,  and  it  is  a good  plan  to  make  all  the  openings — both 
those  into  the  cross  flue  and  those  in  the  division  walls  larger 
than  the  plan  shows  then  close  them  with  loose  bricks  to  the 
proper  size.  Should  the  draft  not  be  uniform,  the  sizes  of 
the  various  openings  can  be  easily  changed  by  the  placement 
or  removal  of  the  throat  bricks.  Thus  the  kiln  can  be  ad- 
justed to  correspond  to  the  conditions,  and  the  adjustment  is 
permanent. 

Fully  perforated  floors  preferably  have  the  collecting  flues 
below  the  floor  flues,  which  gives  opportunity  for  graduated 
openings  from  the  floor  flues  into  the  collecting  flues.  The 
western  kiln,  previously  mentioned,  is  of  this  latter  type  ex- 
cept it  does  not  have  graduated  openings  distributed  over  the 
kiln  area  through  the  diaphragm  between  the  floor  and  the 
collecting  flues,  but  instead  there  is  a single  peripheral  open- 


210 


BURNING  CLAY  WARES. 


in g from  each  perforated  floor  flue  into  each  underlying  col- 
lecting flue.  All  the  gases  escaping  through  the  floor  must 
travel  to  the  kiln  wall,  then  down  into  the  collecting  flues  and 
back  to  the  cross  flue.  In  other  words,  the  draft  is  entirely  in 
the  outer  ring  of  the  kiln. 

There  are  numerous  modifications  of  the  cross  head  flue 
type  of  kiln. 

Fig.  88  shows  a combination  of  a cross  head  flue  with  a 
ring  collecting  flue.  The  cross  flue  is  divided  into  three  parts, 
each  controlled  by  a separate  draft  flue  extended  to  the  out- 
side of  the  kiln,  thence  a single  flue  to  the  stack.  The  claim 
for  this  plan  is  individual  control  of  the  center  and  sides.  The 
design  is  an  effort  to  get  the  distribution  of  multiple  stacks 
with  a single  outside  stack. 

Fig.  89  and  Fig.  90  show  a kiln  presented  by  Greaves- 


Figure  89. 


Walker,  which  may  be  classed  as  a cross  head  flue  type  with 
the  difference  that  both  the  main  flue  and  cross  flue  are  con- 
nected with  the  collecting  flue  system.  The  features  of  the 
kiln  are,  the  sloping  collecting  flues  as  in  the  kiln  illustrated 
in  Fig.  75,  and  the  large  main  flue,  which  may  be  entered  from 
the  outside  to  remove  the  accumulated  sand  without  disturb- 
ing the  kiln  floor. 

Comparison  of  this  kiln  with  the  two  preceding  kilns  again 
brings  out  a marked  difference  in  the  draft.  The  kiln  being 
considered  has  its  strongest  draft  on  a quarter  point  in  the 
kiln  circumference,  decreasing  in  intensity  toward  the  oppo- 
site quarter,  and  still  further  decreasing  in  intensity  toward 


BURNING  CLAY  WARES. 


211 


the  ends  of  the  cross  flue,  in  consequence  of  the  right  turn.  If 
such  a kiln  will  give  satisfactory  results,  and  it  is  said  to  do 
so,  why  complicate  a kiln  bottom  with  multiple  draft  flues 
and  minute  adjustments  as  in  the  two  preceding  kilns?  We 
believe  that  there  are  extremes  in  complications  of  bottoms 
to  get  theoretically  uniform  heat  distribution,  but  barring  such 
extremes,  the  kiln  with  the  best  distribution  will  give  the  bet- 


ter results,  leaving,  as  it  does,  less  to  the  skill  and  intelligence 
of  the  burner. 

A third  modification  of  the  cross  flue  developed  cn  the  Pa- 
cific coast,  illustrated  by  Fig.  91  and  Fig.  92,  eliminates  the 
blind  draft  flue,  which  is  also  eliminated  in  the  Greaves- 


Figure  91. 


Walker  kiln,  and  instead  of  the  direct  and  transverse  flues, 
uses  two  direct  flues.  This  plan  has  individual  control  of  the 
halves  of  the  kiln,  combining  the  idea  of  individual  control  of 
the  first  mentioned  modification,  with  the  self-cleaning  idea  of 
the  last  mentioned  kiln. 

In  this  kiln  we  have  the  draft  all  from  one  side,  but  it 


212 


BURNING  CLAY  WARES. 


would  be  better  to  have  the  draft  connections  on  opposite  sides 
of  the  kiln,  thus  giving,  in  a measure,  a balanced  draft.  As 
it  is,  the  least  kiln  area  on  one  side  has  a maximum  draft 
and  on  the  other  side  a minimum.  Control  on  opposite  sides 
would  make  both  sides  alike  and  while  not  theoretically  per- 
fect, yet  considering  the  results  obtained  from  the  second  modi- 


fication, it,  with  the  change  suggested,  would  give  practical 
uniformity. 

We  illustrated  an  up-draft  potter’s  kiln  in  Figs.  66  and  67. 
The  central  stack  kiln  as  shown  in  Figs.  77  and  78,  or  with 
other  arrangement  of  under  floor  flue  system,  is  considered  by 
many  the  best  type  of  kiln  for  pottery  work. 

The  bottom  of  a simple  down-draft  potter’s  kiln  with  wall 
stacks  is  shown  in  Fig.  93.  The  under  floor  flues  are  circular 
with  openings  in  the  walls  to  form  radial  flues  on  the  same 


level,  leading  to  the  wall  stack  flues,  as  in  the  Eudaly  kiln. 
This  kiln  is  practically  a Eudaly  kiln  adapted  to  pottery  work 
and  the  kiln  properly  belongs  with  the  multiple  wall  stack 
kilns  except  that  individual  stacks  are  not  built  on  the  kiln 


BURNING  CLAY  WARES. 


213 


wall,  but  instead,  a large  stack,  the  full  diameter  of  the  kiln 
at  its  base  tapering  to  the  requisite  opening  in  the  top,  is 
used.  Such  a stack  is  shown  in  Fig.  66  and  Fig.  94.  The  lat- 
ter illustration  together  with  Fig.  95,  is  from  a kiln  designed 


Figure  94. 


by  C.  B.  Harrop.  The  plan,  Fig.  95,  shows  three  levels,  one 
through  the  lower  draft  flues,  one  through  the  floor  flues,  and 


Figure  95. 


one  above  the  floor  level.  The  vertical  view  shows  a section 
through  a furnace  and  one  through  a draft  flue.  The  waste 
gases  circulating  through  the  upper  flue  level  are  drawn  to 
the  center,  then  down  and  out  to  the  wall  flues  through  the 


214 


BURNING  CLAY  WARES. 


lower  level.  The  floor  openings  are  graduated  in  size,  being 
smallest  near  the  center,  to  give  uniform  draft  over  the  floor 
area.  Prof.  Harrop  suggests  that  the  floor  system  could  be 
simplified  without  serious  loss  by  taking  the  gases  to  the  wall 
flues  direct  from  the  upper  level,  thus  eliminating  the  lower 
set  of  flues. 

The  large  stack  typical  of  a “bottle”  (pottery)  kiln  is  not 
always  necessary.  For  instance,  in  the  up-draft  kiln,  Fig.  G6, 
a small  central  stack  above  the  crown  of  the  kiln  can  be 
carried  on  I beams  supported  by  the  kiln  walls. 

Banding  Round  Kilns. 

We  haye  differences  of  opinion  in  regard  to  the  proper 
banding  of  a round  kiln,  varying  from  a single  band  at  the 
crown  skew  level  to  a complete  steel  casing,  including  the 
furnaces.  Regarding  the  advantages  of  the  latter,  Prof.  Har- 
rop in  “Kiln  Expansion  and  Bracing,”  says,  in  substance: 

(1)  The  kiln  wTalls  are  kept  plumb. 

(2)  There  is  no  leakage. 

(3)  The  walls  are  protected  from  the  elements. 

(4)  Steel  radiates  12  per  cent,  less  heat  than  fire  bricks. 

No  one,  we  think,  will  question  the  advantage  of  a steel 

casing  and  only  the  cost  of  installation  prevents  its  universal 
adoption. 

The  usual  thought  in  relation  to  kiln  bands  is,  that  the  chief 
purpose  is  to  hold  the  walls  in  place  and  prevent  the  collapse 
of  the  crown,  for  this  we  do  not  need  steel  casing.  Harrop 
figures  that  a band  y8  inch  by  6 inches  is  strong  enough  to 
withstand  the  thrust  of  a 30-foot  kiln  crown,  but  unless  the 
kiln  walls  are  properly  laid  to  take  care  of  the  wall  expansion, 
the  strain  on  the  band  may  be  greater  than  that  due  to  the 
crown  thrust.  We  have  had  5-16  inch  by  6 inch,  and  % inch 
by  10  inch  bands  burst  on  smaller  kilns  than  30  feet  diameter. 

We  found  it  necessary,  where  the  banding  consisted  of  sev- 
eral narrow  bands,  to  adjust  the  tension  during  the  first  heat- 
ing up  of  the  kiln.  At  intervals  of  about  30  minutes  each  band 
was  loosened  to  a normal  tension  and  thus  we  provided  for  the 
wall  expansion.  After  the  kiln  cooled,  we  found  all  the  bands 
loose,  showing  that  the  kiln  wall  practically  carried  the  crown 
thrust. 

A very  extensive  practice  is  to  use  a single  band  24  inches 
to  36  inches  wide  as  the  height  of  the  wall  above  the  kiln  door 
will  permit,  sometimes  cutting  out  the  lower  side  of  the  band 


BURNING  CLAY  WARES. 


215 


to  conform  to  the  door  arch.  Such  a band  is  put  in  place  when 
the  wall  has  reached  the  proper  height,  fully  riveted,  thus 
eliminating  bolts  and  lugs,  and  the  wall  is  carried  up  to  the 
final  height  inside  the  band.  Such  a band  is  amply  strong  for 
both  crown  thrust  and  wall  expansion. 

It  is  questionable  however,  whether  such  a band  is  the  best 
practice.  It  is  good  in  that  it  gives  a wide  margin  of  safety, 
and  in  so  far  as  it  approximates  a steel  casing,  but  unless  it  is 
supplemented  by  lower  bands  the  walls  will  bulge  and  crack, 
thus  opening  passage  ways  for  cold  air  in  the  lower  part  of  the 
kiln,  where  the  inward  leakage  is  greatest  and  where  (in  down- 
draft  kilns)  we  have  the  greatest  difficulty  in  attaining  the  de- 
sired temperature. 

It  would  be  better  practice  to  case  the  lower  part  of  the 
kiln  wall  with  a wide  band  and  support  the  crown  with  a nar- 
row band,  but  since  the  ash  pit,  fire  doors,  and  wicket  opening 
complicate  the  use  of  a wide  band  around  the  bottom,  it  is 
more  practical  to  use  a number  of  narrow  bands  which  can  be 
placed  to  miss  the  openings.  One  band  can  be  placed  with  its 
top  edge  flush  with  the  kiln  floor  or  ash  pit  floors.  A second 
band  can  be  placed  at  the  grate  bar  level.  A third  band  at 
the  top  of  the  fire  mouth,  and  above  this  other  bands  can  be 
spaced  at  close  intervals  up  to  and  including  the  crown  skew 
level.  The  bands  below  the  wicket  arches  can  be  carried 
across  the  wicket  opening  by  long  bolts,  or  better,  a steel 
frame  can  be  placed  at  the  wicket  jambs,  and  the  bands  within 
the  wicket  height  attached  to  this  frame.  Such  a frame  can 
be  made  of  wide  channel  bars  set  flush  with  the  wicket  jambs 
and  riveted  to  the  bands  below  the  floor  level  and  above  the 
wicket  arch,  and  to  these  channels  the  intermediate  kiln  bands 
can  be  riveted,  thus  leaving  the  wickets  clear. 

The  bands  should  be  spaced,  so  far  as  possible,  to  include 
every  course  of  bricks  as  shown  in  Fig.  96,  and  where  this  is 
not  possible,  as  in  the  lower  kiln  wall,  vertical  bands  spaced 
24  inches  to  36  inches  as  shown  in  Fig.  97,  should  be  used.  In 
one  instance  a kiln  was  cased  with  12  inch  bricks  on  end  and 
the  joints  covered  with  a narrow  band  as  shown  in  Fig.  98. 

A kiln  properly  banded  will  require  9 to  13  bands  and,  con- 
sidering the  cost  of  a kiln,  and  the  losses  resulting  from  a 
distorted  wall,  one  can  readily  afford  proper  bands,  if  indeed 
he  cannot  go  further  and  completely  case  the  kiln  wall  with 
steel,  and  back  up  the  casing  with  a porous  insulating  brick. 


216 


BURNING  CLAY  WARES. 


and  finally  cover  the  crown  deeply  with  ashes  or  some  better 
material. 

Such  a kiln  construction  will  result  in : 

(1)  Direct  fuel  economy. 

(2)  Quicker  bums  with  further  fuel  saving  and  less  labor. 

(3)  Better  results. 

Many  clayworkers  save  a little  in  the  cost  of  installation 


Fig.  96.  Fig.  97.  Fig.  98. 


and  then  for  all  time  complain  that  the  cost  of  burning  and 
upkeep  eats  up  all  the  profits  of  the  business. 

Up-and-Down  Draft  Kilns. 

There  have  been  a number  of  up-and-down-draft  kilns  de- 
veloped, but  as  a rule  they  have,  not  shown  sufficient  merit 
over  the  simple  up-draft  or  down-draft  types  to  lead  to  any 
wide  use,  except  in  some  one  particular  line  of  ware  for  which 
the  kiln  was  developed. 

The  chief  difficulty  with  them  has  been  that  the  ware  in 
the  bottom  of  the  kiln  was  subjected  to  the  maximum  flame 
temperature,  resulting  in  overburning  the  bottom  courses  of 
ware  or  if  not  overburning,  at  least  discoloring  in  consequence 
of  flame  contact  and  reduction.  Another  difficulty  in  some  of 
the  types  has  been  to  maintain  the  combustion  flues,  subjected 
as  they  were  to  such  intense  temperature  and  constructed  of 
the  usual  kiln  fire  brick,  often  inferior  in  quality. 

The  early  up-and-down-draft  attempts  were  very  simple,  as 
shown  in  Fig.  99,  adapted  to  a rectangular  kiln.  An  under 
floor  flue  connected  the  furnace  with  the  interior  of  the  kiln, 


BURNING  CLAY  WARES. 


217 


and  a damper  controlled  the  bag  outlet  from  the  furnace.  With 
this  damper  closed,  and  the  throat  to  the  under  floor  flue  open, 
the  gases  passed  under  the  floor,  then  up  through  the  ware 
until  caught  and  drawn  down  by  the  force  of  the  draft.  The 
draft  flues  might  be  parallel  to  the  combustion  flues  extending 
from  the  bag  wall  to  the  center  flue,  instead  of  a single  central 
draft  flue  with  perforated  top  as  illustrated. 

Round  kilns  work  out  equally  well  with  a series  of  radial 
flues — one  set  from  the  furnaces  to  the  center  for  combustion 
flues,  and  the  alternate  flues  from  midway  between  the  bags 
to  the  center  for  the  draft  flues,  connecting  with  a center  well, 
center  stack,  or  wall  stacks. 


The  throat  to  the  under  combustion  flue  could  be  easily 
fully  closed  with  a fire  clay  block,  or  partially  with  fire  bricks, 
as  the  up-draft  feature  might  require. 

Up-draft  water  smoking  is  considered  the  most  desirable, 
and  a number  of  down-draft  kilns  have  been  designed  for  such 
up-draft  work  to  be  followed  by  down-draft  burning. 

The  original  Eudaly  kiln,  Fig.  80,  had  this  feature.  The 
radial  flues  were  extended  through  the  kiln  walls  and  boxed 
in  outside  the  walls.  By  closing  the  stack  dampers,  furnace 
doors  and  ashpits,  opening  the  crown  vent,  and  using  the  open 
ends  of  the  radial  flues  as  furnaces  or  air  inlets  the  operation 
became  up-draft,  either  for  watersmoking  or  cooling  the  kiln, 


Fig.  99, 


Fig.  100. 


218 


BURNING  CLAY  WARES. 


but  so  far  as  our  observation  goes,  little  use  was  made  of  this 
feature  of  the  kiln. 

Any  radial  or  cross  head  flue  kiln  could  be  adapted  to  this 
purpose,  and  several  kilns  offered  to  clayworkers  have  had 
the  up-draft  water  smoking  feature. 

So  far  as  the  up-draft  feature  in  the  burning  is  concerned, 
practically  the  same  results  are  obtained  by  openings  through 
the  lower  part  of  the  bag  wall,  and  we  frequently  find  such 
bag  wall  construction,  generally,  however,  by  building  the  bag 
wall  in  open  checker  work. 

The  Stewart  kiln  was  a popular  one  in  drain  tile  burning 
and  to  some  extent  in  other  lines  of  ware.  Had  the  kiln  gen- 
erally been  built  of  better  material  its  use  would  have  con- 
tinued and  been  extended,  but  many  installations  were  built 


Figure  101. 


of  very  inferior  fire  bricks  instead  of  the  best  obtainable,  and 
in  consequence  there  were  many  complaints  in  regard  to  the 
failure  of  the  combustion  flues  after  a short  period  of  use, 
sometimes  in  the  first  bum. 

Fig.  100  and  Fig.  101  illustrate  the  principle  of  this  kiln. 

The  furnaces  are  on  opposite  sides  of  the  kiln  and  stag- 
gered to  bring  them  in  line  with  the  combustion  flues.  Each 
furnace  has  two  flues  leading  from  it,  under  the  floor,  to  a 
bag  wall  on  the  opposite  side.  The  adjacent  pair  of  flues  leads 
from  a furnace  on  the  opposite  side  to  a bag  across  the  kiln, 
and  thus,  the  floor  system  consists  of  alternate  pairs  of  flues 
carrying  gases  in  opposite  directions,  each  pair  having  its  fur- 
nace and  corresponding  bag.  The  floor  is  solid  throughout  and 


BURNING  CLAY  WARES. 


219 


the  outlets  from  the  kiln  to  the  wall  stacks  are  in  the  kiln 
wall  at  or  near  the  floor  level. 

The  ware  in  the  lower  part  of  the  kiln  is  burned  by  heat 
conducted  through  the  floor,  and  the  ware  in  the  upper  part 
of  the  kiln  by  direct  contact  with  the  gases  from  the  bags. 
It  could  hardly  be  called  an  up-and-down-draft  kiln,  but  the 
effect  was  the  same  in  that  the  top  and  bottom  were  burned 
at  the  same  time,  and  equally  hard  burned.  The  ware  which 
receives  the  least  heat,  and  which  is  the  last  to  finish  burning, 
is  in  or  near  the  vertical  center  of  the  kiln. 

The  kiln  was  first  designed  and  most  generally  built  as  a 


Figure  102. 


round  kiln  to  which  the  principle  is  less  adapted  than  to  a 
rectangular  shape.  In  the  round  kiln,  it  was  necessary  to  set 
the  ware  with  flues  in  the  bottom  to  insure  any  degree  of  uni- 
form draft  over  the  kiln  area.  This  feature  was  decidedly  ob- 
jectionable especially  for  drain  tile  for  which  the  kiln  was 
largely  used. 

The  principle  works  out  nicely  in  the  rectangular  kiln  by 
alternating  the  furnaces  in  singles  or  in  pairs  with  a cross 
draft  flue  to  wall  stacks  as  shown  in  Fig.  102.  This  enables  us 
to  build  the  kiln  any  desired  length  for  large  capacities,  which 
was  not  possible  in  the  original  kiln,  with  its  fully  solid  floor 
and  stack  inlets  above  the  floor  level.  We  found  it  possible  in 
such  a rectangular  kiln,  to  burn  entire  kilns  of  special  ware 


220 


BURNING  CLAY  WARES. 


which  in  the  regular  down-draft  kiln  could  only  be  burned  in 
the  upper  part  of  the  kiln,  and  no  special  setting  was  required 
in  the  bottom  as  in  the  original  kiln. 


Above  Fig.  103.  Below  Fig.  104. 


A recent  up-and-down-draft  kiln  has  a unique  feature  which 
may  overcome  the  difficulty  of  the  earlier  types.  In  the  latter 
the  up-and-down-drafts  were  obtained  by  a distribution  of  the 


BURNING  CLAY  WARES. 


221 


heat  from  one  set  of  furnaces,  and  the  intense  heat  essential 
for  down-draft  had  a serious  effect  on  the  ware  when  diverted 
to  the  up-draft,  especially  in  view  of  the  proximity  of  the  ware 
to  the  furnace. 

The  kiln  in  question,  shown  in  Fig.  103  and  Fig.  104,  uses 
two  sets  of  furnaces,  one  for  up-draft  and  one  for  down-draft, 
and  each  may  be  fired  as  the  conditions  require.  The  gases 
from  the  furnaces  for  up-draft  are  led  through  main  flues  and 
distributed  in  cross  flues  with  perforated  covers  (kiln  floor) 
throughout  the  under  floor  system,  then  rise  into  the  kiln  and 
come  in  contact  with  the  ware. 

The  stacks  are  approximately  on  the  quarters  of  the  kiln, 
and  the  inlets  from  the  kiln  are  in  the  kiln  wall  three  or  more 
feet  above  the  floor.  The  down-draft  gases  follow  the  usual 
course,  up  through  bags  and  down  through  the  ware,  except 
that  they  do  not  pass  to  and  through  the  floor  as  in  an  ordinary 
down-draft  kiln,  but  instead  are  drawn  off  to  the  stack  inlet 
above  mentioned.  The  up-draft  furnaces  may  be  used  for  up- 
draft water  smoking  by  closing  the  down-draft  furnaces,  open- 
ing the  crown  vent  and  closing  the  stack  dampers.  The  illus- 
tration shows  the  up-draft  furnaces  in  the  base  of  the  stack 
and  by  discontinuing  the  use  of  the  stack  furnaces  except  for 
draft  intensity,  with  proper  damper  adjustment  the  burning 
operation  could  be  entirely  down-draft.  The  course  of  gases 
from  the  down-draft  furnaces  would  be,  up  through  the  bags, 
down  through  the  ware  and  perforated  floor,  into  the  collect- 
ing flues,  thence  to  the  main  cross  flues,  through  the  up-draft 
furnaces,  and  up  through  the  stacks.  The  furnace  in  the  stack 
base  with  the  connecting  damper  gives  opportunity  to  heat  the 
stack  for  increased  draft  to  any  degree  and  at  any  time  during 
the  burning  operation. 

The  Gamble  and  Bryan  up-and-down-draft  pottery  kiln  is 
illustrated  in  Fig.  105  and  Fig.  106.  In  general  the  firing  is 
that  of  a simple  up-draft  pottery  kiln  as  shown  in  Fig.  66  and 
Fig.  67,  and  it  could  be  used  exclusively  for  up-draft  wmrk  with 
the  crown  vent  open  and  the  wall  flue  dampers  closed.  Be- 
tween the  radial  flues  from  the  furnaces  is  a duplicate  set  of 
radial  flues,  with  floor  inlets,  connected  with  the  wall  flues. 
It  is  only  necessary  to  close  the  crown  vent  and  open  the  wall 
flue  dampers  to  convert  the  operation  to  down-draft. 

Another  similar  up-and-down-pottery  kiln  has  the  stacks  for 
down-draft  operation  outside  the  kiln  wall,  but  leading  into  the 


222 


BURNING  CLAY  WARES. 


Above  Fig.  105.  Below  Fig.  106. 


kiln  main  stack  above  the  kiln  crown  and  connected  at  the  base 
with  the  bottom  of  the  kiln. 

We  have  mentioned  the  difficulty  of  maintaining  under  floor 
flues  for  burning  bricks  by  up-draft,  but  it  must  be  noted  that 


BURNING  CLAY  WARES. 


223 


pottery  kilns  are  largely  of  this  type,  without  serious  failure 
in  this  respect,  although  the  temperatures  required  are  much 
higher.  The  only  explanation  is  that  pottery  kilns  are  built  of 
better  material.  The  comparison  in  favor  of  the  pottery  kiln 
might  be  carried  still  further,  for  one  has  only  to  compare 
pottery  and  muffle  kilns  with  the  average  brick  and  tile  kiln  to 
note  the  difference  in  the  structures  in  every  particular.  If  pot- 
tery kilns  were  built  of  the  same  materials  used  in  common 
ware  kilns,  the  pottery  industry  would  be  in  a sad  plight. 

Horizontal  Draft  Kilns. 

A description  of  a horizontal  draft  periodic  kiln  would  not 
be  necessary  except  for  our  classification  of  kilns,  because  such 
kilns  find  little  use  in  this  country.  We  do  not  know  of  any 
distinctively  horizontal-draft  kiln  at  the  present  time,  but  we 
have  seen  one  or  two  such  kilns  in  the  past  which  have  since 
been  abandoned. 

It  is  an  early  type  of  crowned  kiln  and  perhaps  a natural 
adaptation  of  a furnace,  a hearth  and  a stack. 

Fig.  107  illustrates  an  early  horizontal-draft  kiln  and  little 
explanation  is  needed.  There  are  three  or  more  furnaces  in 
the  front,  one  of  which  is  in  the  doorway  and  after  each  burn 
is  torn  out  to  provide  an  entrance  to  the  kiln.  The  kilns 
were  built  tapering  on  the  sides  and  crown  from  front  to 
stack  presumably  on  the  principle  that  as  the  gas  gave  up  its 
heat,  became  lower  in  temperature  and  less  in  volume,  the 
mass  of  ware  to  be  burned  decreased  correspondingly.  One 
would  think  that  the  reverse  would  be  the  proper  principle 
wherein  the  high  temperature  gas  and  large  volume  would 
quickly  pass  the  restricted  mass  of  ware,  leaving  a greater 
volume  of  heat  and  a slower  movement  for  the  larger  mass 
of  ware.  Whatever  the  correct  principle  the  horizontal-draft 
kiln  has  little  merit. 

The  nearest  approach  to  a horizontal  draft  is  a kiln  used  in 
burning  fire  bricks,  shown  in  Fig.  108.  It  is  properly  a down- 
draft  kiln  but  an  effort  is  made  to  get  a horizontal  draft. 
It  is  equivalent  to  two  horizontal  kilns  placed  back  to  back. 
In  each  end  are  three  furnaces,  one  being  in  the  doorway. 
The  bags  are  low  and  built  of  wide  open  checker  work  for 
the  horizontal  draft.  Midway  in  the  kiln  is  a cross  main  draft 
flue  leading  to  a stack  outside  the  kiln  wall. 

The  kilns  may  be  built  singly,  but  usually  they  are  in  bat- 
teries of  several  kilns  in  touch  with  each  other  and  it  is  only 


224 


BURNING  CLAY  WARES. 


necessary  to  brace  the  outside  walls  of  the  end  kilns.  A fur- 
ther advantage  of  the  battery  construction  is  the  elimination 
of  some  radiation  loss  from  the  sides  of  the  kilns.  The  kilns 
are  necessarily  limited  in  capacity,  and  size  considered  it  is 
not  possible  to  get  as  uniform  temperature  throughout  the 
kiln  as  in  the  more  modern  down-draft  kilns.  The  best  evi- 
dence of  the  inefficiency  of  the  kiln  is  that  the  newer  fire 
brick  plants  controlled  by  the  same  corporations  have  built 
the  round  down-draft  kiln. 

It  has  been  our  opinion  for  a long  time  that  some  type  of  a 
continuous  kiln  had  its  most  promising  field  in  the  manu- 


facture of  fire  bricks  because  of  the  high  temperatures  re- 
quired and  excessive  fuel  loss  in  consequence,  but  there  has 
been  very  little  development  along  this  line.  One  factory 
used  for  many  years  a gas-fired  semi-continuous  ring  (tunnel) 
kiln,  now  dismantled,  and  another  factory  built  two  gas-fired 
chambered  kilns  of  a Scotch  design,  but  abandoned  them  after 
a short  period.  Also  a small  ring  kiln,  direct  coal  fired,  on 
an  Illinois  fire  brick  yard  has  been  wrecked  after  many  years 
use. 

In  spite  of  the  several  attempts  to  use  regenerative  kilns  in 
this  industry  and  the  unsatisfactory  results,  we  do  not  be- 
lieve this  type  of  kiln  has  had  a fair  trial  in  this  line,  and  we 
still  hold  the  same  opinion  in  regard  to  the  usefulness  of  the 
continuous  kiln  in  the  fire  brick  industry. 


BURNING  CLAY  WARES. 


225 


At  the  present  time  three  car  tunnel  kilns  are  being  tried 
out  in  fire  brick  manufacture,  two  of  which  are  proven  to  be 
successful,  and  it  is  to  be  hoped  that  the  problem  of  fuel 
economy  in  fire  brick  burning  has  been  solved  in  these  kilns. 

Muffle  Kilns. 

Muffle  kilns  are  used  to  burn  terra  cotta,  enameled  bricks, 
and  other  wares,  which  must  be  protected  from  contact  with 
the  furnace  gases,  and  in  consequence  burned  by  radiant  heat 
from  a muffle  wall  separating  the  ware  chamber  and  furnace 
flues. 

It  is  apparent  that,  to  be  economical,  the  muffle  walls  must 
be  as  thin  as  possible  and  the  arrangement  of  the  kiln  ducts 


must  provide  for  a complete  encircling  of  the  muffle  by  the 
furnace  gases.  These  are  the  problems  of  the  muffle  kiln. 

The  up-draft  muffle  kiln  needs  no  illustration.  If  we  take 
an  up-draft  pottery  kiln  such  as  that  shown  in  Figs.  66  and 
67,  and  build  a muffle  inside  of  it,  we  will  have  an  up-draft 
muffle  kiln. 

If  the  kiln  is  small,  no  center  flue  is  required,  but  we  retain 
the  under  floor  flues  connected  with  the  furnaces  although 
there  is  no  positive  draft  movement  under  the  floor.  These 
flues  extend  across  the  kiln  from  furnace  to  furnace  and  the 
floor  is  heated  by  radiant  heat  from  the  furnaces  and  by  con- 
vection through  pulsations  of  the  gases  as  the  pressure  condi- 
tions in  the  several  furnaces  vary  from  time  to  time.  When 


226 


BURNING  CLAY  WARES. 


there  is  a large  volume  of  gas  in  one  furnace  the  pressure 
forces  some  of  the  gas  under  the  floor,  perhaps  fully  across 
to  the  opposite  side,  until  the  pressure  all  around  is  equalized. 

The  central  opening  or  any  openings  between  the  under 
floor  flues  and  the  kiln  chamber  of  the  pottery  kiln  are  closed 
when  converted  into  a muffle  kiln,  and  the  bag  walls  are  con- 
verted to  an  annular  ring  carried  nearly  to  the  height  of  the 
kiln  wall  and  completed  with  a crown  a few  inches  below 
the  kiln  crown.  The  movement  of  the  furnace’s  gases  is  up 
through  the  annular  space  between  the  muffle  wall  and  kiln 


wall,  then  through  the  space  between  the  muffle  crown  and 
kiln  crown  to  a central  vent  in  the  kiln  crown. 

The  stack  may  be  the  usual  pottery  stack  resting  on  the 
kiln  wall  and  tapered  to  the  required  opening  at  the  proper 
height,  or  it  may  be  a smaller  structure  supported  by  I beams, 
or  by  the  kiln  crown,  or  by  a third  crown  sprung  from  the  kiln 
walls  for  this  special  purpose.  The  latter  mentioned  support 
is  illustrated  in  Fig.  115. 

For  large  up-draft  kilns,  the  central  opening  in  the  floor  of 
the  pottery  kiln  is  retained  and  extended  to  the  top  of  the 
muffle  crown  by  a circular  wall.  The  movement  of  the  gases 
under  the  floor  and  up  through  the  central  flue  is  positive,  and 
the  mass  of  ware  within  the  muffle  is  heated  by  conduction 
through  this  flue  wall  and  radiation  from  it  to  the  ware  as 


BURNING  CLAY  WARES. 


227 


well  as  by  conduction  through  and  radiation  from  the  circum- 
ferential muffle  wall.  Such  up-draft  designs  are  preferable 
for  small  kilns,  particularly  for  short  flame  fuels. 

A rectangular  muffle  kiln  developed  in  England  and  used  in 
Canada  in  the  manufacture  of  enameled  bricks  is  illustrated 
in  Fig.  109,  Fig.  110,  and  Fig.  111.  Fig.  109  is  a vertical  section 
through  the  furnace  and  Fig.  110  is  a corresponding  section 
through  the  flue  adjacent  to  the  furnace.  Fig.  Ill  is  a com- 
bined view  plan  below  and  above  the  floor. 

The  gases  from  the  furnaces  pass  under  the  muffle  floor 
to  the  opposite  side,  then  up  through  a flue  space  between  the 


Figure  111. 

muffle  wall  and  kiln  wall,  and  this  flue  continues  over  the 
muffle  crown  and  down  to  the  floor  on  the  furnace  side.  When 
the  gases  reach  this  point  they  pass  through  an  opening  into 
the  adjacent  parallel  flue,  then  rise,  pass  over  the  muffle 
crown,  and  down  to  the  floor  on  the  opposite  side  where  they 
escape  into  the  wall  draft  flue,  up  and  out  into  the  stack.  Thus 
the  gases  make  a complete  circuit  of  the  muffle  and  reverse 
except  they  do  not  pass  under  the  floor  a second  time,  but  in- 
stead are  drawn  off  into  the  stack  at  the  floor  level. 

The  muffle  floor  support  consists  of  a number  of  single 
brick  piers  which  permit  complete  circulation  under  the  floor, 


228 


BURNING  CLAY  WARES. 


and  this  circulation  may  be  controlled  by  dampering  the  indi- 
vidual wall  draft  flues. 

The  kiln  is  necessarily  narrow  to  insure  a substantial 


Figure  112. 

muffle  construction.  The  construction  is  simple  and  it  has 
been  proven  practical  to  reduce  the  thickness  of  the  muffle 
walls  to  1V2  inches. 

A down-draft,  or  one  might  call  it  an  up-and-down-draft, 


Figure  113. 

muffle  kiln  is  illustrated  in  Fig.  112,  which  shows  the  plan 
through  the  lower  return  flues  and  that  above  the  muffle 
floor.  Fig.  113  is  a vertical  section  through  a furnace  on  one 
side  and  a doorway  on  the  opposite  side.  The  latter  shows 


BURNING  CLAY  WARES. 


the  stack  within  the  central  muffle  flue,  but  this  construction 
is  not  essential.  After  the  gases  have  reached  the  inlet  at 
the  base  of  the  stack,  they  may  be  drawn  off  through  the 
stack  as  shown,  or  drawn  down  into  a central  well,  and  to 
an  outside  stack  or  fan  through  an  underground  draft  flue. 
The  situation  is  the  same  as  that  of  a central  stack  and  central 
well  open  fire  periodic  down-draft  kiln,  where  either  plan  of 
removing  the  gases  may  be  used,  and  both  methods  are  used 
in  the  muffle  construction. 

The  circular  feather  floor  walls  shown  in  the  plan  provide 
direct  flues  to  the  center,  but  above  the  diaphragm  these  walls 
are  staggered  as  shown  in  the  vertical  section,  and  the  gases 
in  their  passage  from  the  center  to  the  circumference  must 
take  a sinuous  course  before  dropping  into  the  lower  direct 
flues.  This  insures  full  contact  with  the  muffle  floor  and  a 
maximum  absorption  of  heat.  The  stack  is  shown  as  a con- 
tinuous structure  from  the  kiln  bottom,  but  the  usual  method 
is  to  independently  support  the  portion  of  the  stack  above 
the  kiln  crown  as  previously  noted. 

The  movement  of  gases  from  the  furnaces  is  up  the  annular 
space  between  the  muffle  and  the  kiln  wall,  then  over  to  the 
central  annular  space  between  the  muffle  and  stack,  down 
which  they  pass  to  the  flues  below  the  floor,  spread  out  to  the 
circumference  of  the  floor  flues  and  are  drawn  down  and  back 
to  the  stack  or  well  through  the  lower  flue  system. 

A long  flame  coal,  by  means  of  which  we  get  more  or  less 
gas  combustion  throughout  the  circuit,  is  essential  in  securing 
uniform  burns. 

It  is  a question  whether  the  central  stack  is  as  good  as  the 
central  well  with  underground  draft  flue.  It  insures  strong 
draft  almost  from  the  beginning  of  the  burning,  and,  as  shown 
in  the  discussion  of  stacks,  does  not  require  the  height  of  an 
outside  stack.  On  the  other  side  of  the  question,  it  decreases 
the  size  of  the  muffle  or  increases  the  diameter  of  the  kiln  for 
any  given  size  or  muffle.  The  chief  point,  it  seems  to  us,  is 
that  the  gases  have  given  up  their  heat  value  when  they  enter 
the  lower  flues,  and  as  they  rise  through  the  stack,  having  a 
lower  temperature  than  the  descending  gases  in  the  annular 
space  between  the  stack  and  the  muffle,  there  will  be  a flow  of 
heat  from  the  hotter  gases  through  the  stack  wall  and  this 
heat  wflll  be  taken  up  and  carried  away  by  the  stack  gases. 
In  other  words  it  seems  to  us  that  after  the  gases  have  ceased 


230 


BURNING  CLAY  WARES. 


to  be  valuable  for  heating  the  muffle,  that  it  is  a mistake  to 
bring  them  again  in  touch  with  it  where  the  only  effect  would 
be  to  lower  the  temperature  of  the  muffle  gases. 

Another  up-and-down-draft  muffle  kiln  is  shown  in  Fig.  114. 


Figure  114. 


Figure  115. 


In  this  kiln  the  annular  space  between  the  muffle  and  kiln 
wall  is  divided  into  sections,  up  in  front  of  the  furnaces,  as 
shown  in  the  section  through  the  furnace,  and  down  between 


BURNING  CLAY  WARES. 


231 


the  furnaces.  The  gases  from  the  furnaces  will  rise  and  he 
projected  into  the  crown  space,  then  caught  and  be  drawn 
down  through  the  alternate  flues  between  the  furnaces,  pass 
under  the  floor,  up  the  central  flue  and  into  the  stack. 

Fig.  115  shows  two  half  sections  of  a more  complicated  kiln 
with  practically  a double  annular  space.  The  inner  space 
into  which  the  furnaces  deliver  their  gases  is  a complete  circle 
but  the  outer  space  is  from  furnace  to  furnace — practically 
broad  flues  between  the  furnaces.  The  movement  of  the  gases 
is  indicated  by  the  arrows. 

Muffle  Kiln  Construction. 

It  is  evident  that  to  get  a satisfactory  life  from  a muffle 
kiln,  it  must  be  built  of  the  best  materials  laid  up  with  great 
care. 

When  we  consider,  that  the  muffle  walls  are  only  214 


— T 

1 

Sr 

"7 

£7ee 

Figure  116. 


inches  thick  and  the  muffle  crown  not  more  than  four  inches 
thick  and  often  as  thin  as  214  inches,  that  the  latter  is  pierced 
by  a number  of  steam  ports  extending  through  both  of  the 
kiln  crowns,  and  that  it  must  make  and  retain  a close  con- 
nection with  the  muffle  flue,  we  appreciate  the  need  of  the 
best  possible  construction. 

If  the  kiln  is  not  properly  banded  and  its  walls  bulge,  as 
frequently  happens  in  common  ware  kilns,  the  muffle  wall 
will  follow  the  kiln  wall  and  become  cracked  and  distorted 
to  its  ruin.  If  the  fire  bricks  are  not  properly  burned  the 
shrinkage  in  use  will  settle  the  muffle  and  tear  the  crown 
away  from  the  steam  ports.  Should  the  central  flue  not  settle 


232 


BURNING  CLAY  WARES. 


to  the  same  degree,  or  should  settle  at  all  without  being  fol- 
lowed by  the  crown,  there  will  be  a separation  of  flue  and 
crown.  The  average  life  of  a well-built  muffle  is  from  forty 
to  fifty  burns,  and  it  is  remarkable  that  the  structure  should 
remain  intact  this  long. 

A common  method  of  building  the  muffle  wall  is  shown  in 
Fig.  116,  all  the  bricks  being  preferably  special  shapes  unless 
the  diameter  of  the  kilns  is  relatively  large.  The  outer  wall 
is  backed  up  by  the  kiln  wall,  and  cannot  crowd  out  unless 


Plan 

Figure  117. 


El  ev. 

Figure  119. 


the  kiln  wall  gives  way,  nor  can  it  come  in  because  of  the 
circle.  The  inner  wall  cannot  crowd  in  because  of  the  circle, 
but  it  can  work  outward. 

An  improvement  on  such  a wall  is  shown  in  Fig.  117,  where 
instead  of  ordinary  wedge  bricks  for  the  ties,  a special  shape 
with  shoulders  is  used.  An  advantage  of  this  plan  is  that 
inner  and  outer  wall  may  use  the  same  shape,  or  a standard 
brick  if  the  kiln  diameter  is  not  too  small. 


BURNING  CLAY  WARES. 


233 


The  central  flue  may  be  built  of  circle  bricks,  but  a better 
construction  is  illustrated  in  Fig.  119,  showing  a tongued  and 
grooved  circle  block.  This  makes  a strong  structure,  the 
walls  of  which  may  be  very  thin. 

Crowns  are  often  built  of  half  bricks  in  wedge  and  key 
shapes,  but  a thinner  crown  is  possible  by  using  tongued  and 
grooved  blocks  somewhat  similar  to  those  in  Fig.  119.  These 
blocks,  of  course,  are  special  shapes,  and  each  ring  has  a 
different  radial  pitch  which  requires  a special  shape.  If  the 
kilns  are  of  a standard  size  the  repair  supplies  become  a 
simple  matter,  but  with  a number  of  sizes  of  kilns  in  no 
way  standardized  the  problem  of  repairs  is  increasingly  diffi- 
cult. 

A later  method  of  muffle  wall  construction  is  shown  in 
Fig.  118.  The  walls  are  built  of  hollow  tile  made  of  the 
best  grade  of  No.  1 fire  clay. 

The  base  of  the  muffle  wall  in  any  construction  is  built 
thicker  to  withstand  the  cutting  action  of  the  flame  and  slag- 
ging action  of  the  ash.  This  thicker  wall  also  protects  the 
ware  from  the  intense  furnace  heat.  The  thin  muffle  wall  of 
whatever  construction  rests  upon  this  base  wall,  but  for  the 
hollow  tile  wall  a ring  distributing  flue  around  the  base  is 
necessary  to  get  the  gases  fully  around  the  kiln  circle. 


234 


BURNING  CLAY  WARES. 


CHAPTER  IX. 

SOME  NOTES  ON  SETTING. 

WE  HAVE  mentioned  the  setting  and  discussed  it  briefly 
in  connection  with  the  descriptions  of  the  kilns,  and 
we  will  not  here  take  up  the  ordinary  setting,  but 
instead  will  take  up  some  special  features. 

A large  factor  in  common  brick  setting  is  the  possibility 
of  rapid  work  even  at  some  expense  in  quality,  and  for  com- 
mon bricks  the  skintle  method,  or  alternate  headers  and 
stretchers,  has  the  preference. 

Face  bricks,  on  the  other  hand,  require  a setting  which 
will  give  the  maximum  quantity  of  first  quality,  and  the  nat- 
ural color  product  is  set  faced  in  alternate  double  courses  of 
headers  and  stretchers,  while  the  fire  flashed  product  is  set 
flat  and  the  setting  is  such  as  gives  the  most  uniform  ana 
maximum  exposure  of  the  faces  to  the  kiln  gases.  The 
inexperienced  clayworker  does  not  see  any  reason  why  flat 
set  bricks  may  not  be  in  alternate  headers  and  stretchers 
in  single  courses  or  a number  of  courses,  and  he  cannot  under- 
stand the  importance  of  the  complicated  setting  used  in  many 
instances.  We  have  seen  such  simple  alternate  flat  setting 
and  the  results  were  as  unsatisfactory  as  the  setting  was 
primitive. 

The  chief  loss  in  the  kiln  output  either  in  broken  bricks 
or  culls  comes  from  the  binder  courses  and  the  base  and 
cap  courses,  and  the  setting  should  be  worked  out  to  keep 
this  loss  to  a minimum.  Some  brick  products  break  easily 
when  subjected  to  kiln  strains,  or  distort  (kiln  mark)  under 
weight  and  particular  attention  must  be  given  to  their  set- 
ting. We  have  seen  kilns  of  brick  irregularly  bonded  through- 
out and  the  strength  of  the  product  was  such  that  the  whole 
mass  was  drawn  together  in  shrinking  without  material  loss 
in  broken  bricks.  The  manufacturer  of  such  bricks  is  par- 
ticularly fortunate  in  his  material.  Other  clays  develop  such 


BURNING  CLAY  WARES. 


235 


weak  products  that  any  bonding  in  larger  masses  than  three 
brick  benches  or  any  lipping  of  one  brick  on  another,  results 
in  a broken  product. 

In  down-draft  kiln  setting,  where  the  ware  extends  above 
the  bags,  the  front  exposed  to  the  flames  must  be  racked 
back  to  prevent  it  from  drawing  over  and  falling  into  the 
bags,  and  the  open  setting  in  such  fronts  must  be  partially 
or  completely  closed  to  force  the  gases  over  the  top.  Three 
brick  benches,  racking  back  above  the  bags,  and  protection  ot 
the  exposed  fronts,  are  shown  in  Fig.  76. 

Setting  in  up-draft  kilns  has  been  fully  discussed  and 
illustrated  in  connection  with  the  kilns.  In  addition  it  may 
be  mentioned  that  some  small  yards  frequently  burn  drain 
tile  in  such  kilns  along  with  bricks.  The  arches  are  set  with 
bricks  in  the  usual  manner.  Above  the  arches,  brick  and 
drain  tile  are  set  in  alternate  benches,  but  the  tile  benches 
to  preserve  the  continuity  of  the  kiln  walls.  Such  a com- 
bination of  bricks  and  tiles  is  not  to  be  recommended.  The 
are  enclosed  with  the  usual  setting  of  bricks  on  the  heads 
tiles,  because  of  their  thin  walls,  burn  more  quickly  than  the 
bricks,  and  since  the  columns  of  tiles  form  chimneys  with 
relatively  low  resistance,  the  tendency  of  the  draft  is  through 
the  tile  benches,  where  it  is  least  needed  and  most  likely  to 
do  damage.  We  have  seen  warped  overburned  tile  enclosed 
by  benches  01  underburned  bricks.  In  the  setting  of  such  a 
combination  care  should  be  taken  in  setting  the  arches  to 
reduce  the  draft  spaces  in  the  brick  setting  under  the  tile 
benches  and  thus  force  the  gases  into  the  brick  benches* 

Variations  in  down-draft  kiln  brick  setting  should  not  be 
necessary  if  the  kiln  is  properly  designed  and  in  good  con- 
dition. This  has  been  mentioned  in  discussing  down-draft 
kiln  bottoms,  and  a study  of  the  bottoms  will  show  that  in 
some  of  them  it  may  be  very  necessary  to  correct  the  defi- 
ciency of  the  bottom  by  variation  in  the  setting.  It  is  pos- 
sible to  materially  increase  the  degree  of  uniformity  of  the 
burning  by  the  setting,  but  if  the  kiln  bottom  is  of  a good 
type  and  the  clay  has  a fair  burning  range,  there  should 
be  little  need  of  variation  in  the  setting.  We  have  experi- 
mented quite  a little  along  this  line.  Starting  with  uniform 
setting  of  8 on  3 in  a down-draft  kiln  we  change  the  upper 
half  of  the  kiln  to  7 on  3,  on  the  principle  of  less  resistance 
and  a rapid  movement  of  the  gases  in  the  upper  part  of  the 
kiln,  thus  conserving  the  heat  for  the  lower  part. 


236 


BURNING  CLAY  WARES. 


We  next  went  to  the  opposite  extreme  and  set  the  lower 
half  7 on  3,  and  the  upper  8 on  3,  and  topping  out  with  two 
courses  set  practically  tight.  We  provided  a square  vertical 
flue  in  the  center,  and  midway  of  the  height  of  the  setting 
were  distributing  flues  from  the  vertical  flue  to  the  outer 
circle  of  the  setting.  The  principle  involved  was  to  burn  the 
upper  muffled  mass  of  bricks  by  conduction  and  the  lower 
mass  by  convection.  The  heat  distribution  was  satisfactory, 
but  considerable  loss  resulted  from  the  distributing  flues,  and 
also  some  damage  accrued  in  consequence  of  the  excess 
weight  above  and  reduced  support  below.  We  finally  came 
back  to  the  original  setting  of  8 on  3 throughout  the  kiln,  but 
by  this  time  we  had  learned  that  there  were  greater  possi- 
bilities in  the  handling  of  the  fires  and  control  of  the  kiln 
draft  than  in  the  different  methods  of  setting. 


Figure  120 


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Figure  121 


Setting  for  Flame  Effects. 

Bricks  to  be  fire  flashed  are  set  on  the  flat  and  as  above 
noted,  it  is  important  that  the  setting  be  such  as  to  permit 
an  equal  flame  contact  with  the  faces  of  the  bricks.  The 
color  effects  are  very  sensitive  to  the  action  of  the  kiln  gases. 
Where  the  bricks  are  set  in  checker  work  the  gases,  after 
passing  the  checkers,  flare  out  and  produce  a fan-shaped 
flame  effect  on  the  faces  of  the  underlying  bricks. 

Walter  A.  Hull’s  paper,  “On  the  Burning  of  Rough  Tex- 
ture Shale  Bricks,”  in  Vol,  XVI,  Trans.  American  Ceramic 
Society,  should  be  read  by  every  clayworker  interested  in 
rough  texture  face  bricks.  The  paper  is  a detailed  descrip- 
tion of  methods  of  burning  to  get  any  desired  color  effects, 
but  it  incidentally  discusses  the  setting. 

Fig.  120,  plan,  and  Fig.  121,  front  elevation,  show  a simple 
setting  for  flashed  bricks.  As  shown  in  the  plan  the  bench 


BURNING  CLAY  WARES. 


237 


starts  with  a stretcher  and  three  headers,  or  two  headers  for 
a two  and  one-half  brick  bench.  This  plan  is  carried  up  five 
to  six  courses  or  more  as  the  uniformity  of  the  brick  will 
permit  and  then  reversed  for  a duplicate  number  of  courses, 
and  thus  alternating  to  the  top  of  the  setting.  It  is  desirable 
to  have  the  setting  work  out  to  form  independent  columns 
3 *4  bricks  by  3,  or  4 or  5 bricks.  The  plan  shows  a 3*4x5 
brick  column,  but  will  also  work  out  3*4  x 4 with  a little  wider 
spacing  than  that  shown.  Frequently  the  headers  are  set 
part  single  and  part  double,  as  in  Fig.  122 — the  double  courses 
set  back  to  back  give  greater  stability  to  the  column  and 
such  setting  permits  us  to  reduce  the  size  of  the  columns 
from  that  shown.  For  instance,  if  the  third  and  fourth 
courses  from  the  left  in  Fig.  120  are  brought  together,  the 
column  would  work  out  four  stretchers  and  the  setting  would 
be  four  single  headers  alternating  with  one  double  header. 
The  benches  are  spaced  about  2*4  inches,  and  to  stay  them 


4 

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Figure  122 


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Figure  123 


it  is  necessary  to  project  headers  at  intervals  across  this 
space  to  abut  against  the  adjacent  bench,  as  seen  in  Fig.  124, 
and  the  double  courses  are  desirable  for  this  purpose,  in  that 
no  broken  bricks  are  required  to  fill  up  the  space  left  vacant 
by  the  projection. 

In  the  sketches  the  bricks  are  shown  set  tight,  which 
would  give  one  flashed  end  in  each  3*4  brick.  This  is  not  a 
sufficient  proportion  of  quoins  for  many  jobs,  and  the  number 
may  be  increased  to  100  per  cent,  by  separating  the  bricks. 
If  the  header  courses  are  set  away  from  the  stretchers  the 
usual  space  we  will  get  two  flashed  ends  from  each  3*4 
bricks.  If  the  loose  end  header  is  set  away  from  the  adja- 
cent header  we  increase  the  flashed  ends  to  3 out  of  a pos- 
sible 3*4,  and  one  of  them  would  have  both  ends  flashed, 
which  is  necessary  for  pier  and  pilaster  work,  and  the  stretch- 
ers may  be  spaced  as  shown  in  Fig.  123,  thus  giving  a flashed 


238 


BURNING  CLAY  WARES. 


end  on  each  brick.  In  some  instances  where  it  is  desirable 
to  have  the  benches  even  bricks — three  or  four,  as  may  be 
required — the  first  setting  has  a stretcher  course  front  and 
rear  and  second  tier  will  be  all  headers,  thus  alternating  to 
the  top. 

The  setting  shown  in  Figs.  122  to  125,  inclusive,  is  more 
complicated,  but  it  has  greater  stability.  The  bottom  course 
is  set,  as  shown  in  Fig.  122,  merely  to  give  a full  bearing  for 
the  superimposed  stretchers.  This  bottom  course  may  be 
omitted,  and  the  setting  started  with  the  regular  layout,  as 
shown  in  Fig.  123.  This  is  carried  up  four  to  six  or  more 
courses,  then  comes  a single  tie  course,  as  shown  in  Fig.  125 
or  Fig.  126,  and  if  Fig.  125  is  the  plan  followed,  this  is  topped 
by  a single  course  reversing  Fig.  122.  Then  follow  four  to 
six  courses,  as  in  Fig.  123  reversed.  The  bench  tie  course, 
Fig.  124,  or  Fig.  125,  may  be  repeated  at  this  level,  or  may 


be  omitted  until  some  higher  level  is  reached,  depending  upon 
the  need  for  ties. 

The  complete  setting  will  be  as  follows : First  course  as  in 
Fig.  122 ; 2nd,  3rd,  4th  and  5th  courses  as  Fig.  123 ; 6th  course, 
Fig.  125;  7th  course,  Fig.  122  reversed;  8th,  9th,  10th  and 
11th  courses,  Fig.  123  reversed;  12th  course,  Fig.  125  re- 
versed. An  occasional  header  is  projected  from  the  plan  Fig. 
122,  or  Fig.  123,  as  shown  in  Fig.  124.  The  12th  course  com- 
pletes the  cycle,  and  the  order  is  repeated  to  the  full  height 
of  the  setting. 

If  we  use  the  tie  shown  in  Fig.  126,  the  setting  will  be  as 
follows:  First  course,  Fig.  122;  2nd,  3rd,  4th  and  5th  courses, 
Fig.  123;  6th  course,  Fig.  126;  7th,  8th,  9th,  10th  and  11th 
courses,  Fig.  123  reversed;  12th  course,  Fig.  126,  if  close  tying 
is  necessary,  or  if  not,  then  the  12th  course  is  as  Fig.  122, 
followed  by  Fig.  123  in  the  13th,  14th,  15th  and  16th  courses, 
then  perhaps  the  tie,  Fig.  126,  followed  by  Fig.  123  reversed. 


BURNING  CLAY  WARES. 


239 


A simple  setting  for  flashed  brick  is  shown  in  Fig.  127, 
front  view,  and  Fig.  128,  side  view.  The  setting  is  started  in 
two  courses  set  on  edge.  Flat  setting  is  carried  up  in  two 
brick  benches  and  the  brick  are  spaced  so  the  flat  setting 


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Figure  126 


will  break  joints  with  the  tie  stretchers.  This  is  also  very 
open  setting  and  expensive  in  kiln  room,  but  the  loss  in  this 
way  may  be  partially  offset  by  quicker  burning.  The  ties  run 


Fig.  127,  Front  View.  Fig.  128,  Side  View- 


through  the  full  width  of  the  setting,  and  as  they  always 
result  in  culls,  it  is  important  that  the  flat  courses  in  each  tier 
be  set  as  high  as  practical  to  reduce  the  number  of  tie  courses 
to  a minimum. 


240 


BURNING  CLAY  WARES. 


About  one-third  and  three-fourths  the  total  height  of  the 
setting,  or  as  frequently  and  at  such  heights  as  may  be  found 
necessary,  a double  tie  course  is  introduced — one  the  regular 
cross  tie  and  the  other  a through  bench  tie,  as  shown  in  Fig. 
128.  Usually  these  bench  ties  are  placed  at  levels  convenient 
for  the  setters— the  first  tie  coming  at  the  limit  height  of 
setting  from  the  floor  level,  and  upon  this  tie  the  setters 
stand  to  carry  the  setting  to  the  higher  levels.  The  ten- 
dency to  rolling  is  greatest  at  the  top  of  the  setting  and  the 
upper  bench  tie  should  be  near  the  top,  even  though  three 
bench  ties  may  be  necessary,  but  a third  bench  tie  is  objec- 
tionable from  a working  standpoint,  because  it  introduces 


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Fig.  129,  Front  View.  Fig.  130,  Side  View 


another  set-back  over  which  the  setters  must  reach  to  set 
the  top  tiers. 

A setting  similar  to  the  preceding  is  shown  in  Fig.  129, 
front  view  ,and  Fig.  130,  side  view.  As  in  the  preceding  the 
flat  courses  are  all  headers,  but  by  using  a double  tie  course 
for  each  tier  the  spacing  is  much  closer.  The  double  tie 
course  gives  greater  stability,  and  is  preferable  where  the 
rolling  and  twisting  tendency  is  excessive,  although  the  num- 
ber of  culls  is  increased.  However,  considering  the  total 


BURNING  CLAY  WARES. 


241 


mass,  the  proportion  of  culls  in  this  setting  will  be  only 
slightly  greater  than  that  in  the  preceding  method. 

A method  of  setting  in  bungs  for  salt  glazing  is  shown  in 
Fig.  131,  front  view  and  Fig.  132  plan.  The  bungs  are  tied 
together  by  headers  spanning  the  intervening  space.  Such 
setting  greatly  reduces  the  capacity  of  the  kiln,  but  for  salt 
glazing  the  results  are  very  satisfactory. 

We  frequently  have  to  burn  bricks  set  flat,  such  as  orna- 


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Above,  Fig.  131,  Front  View. 
Below,  Fig.  132,  Plan. 


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Above,  Fig.  133,  Front  View. 
Below,  Fig.  134,  Plan. 


mental  shapes,  enameled  bricks,  etc.,  the  faces  of  which  must 
be  protected  from  the  flames,  and  the  usual  “boxing  in” 
method  for  such  work  is  illustrated  in  Fig.  133,  front  view  and 
Fig.  134  view  plan.  Each  bench  is  started  with  a course  of 
bricks  on  edge,  and  on  this  is  set  a flat  face  and  tie  course  of 
standard  bricks,  as  shown  in  the  plan  to  carry  and  protect 
the  special  shapes.  Each  tie  course  is  the  same  as  that  shown 
in  the  plan.  The  special  shapes,  or  enameled  bricks,  are  set 
with  their  faces  toward  each  other,  and  only  the  backs  are 


242 


BURNING  CLAY  WARES. 


exposed  in  the  larger  draft  spaces  between  each  double  tier. 
The  benches  are  set  in  close  touch  with  each  other,  and  may 
be  tied  together  to  maintain  ciose  contact,  ana  tnus  prevent 
the  gases  from  passing  down  between  the  benches  and  baf- 
fling back  into  the  narrow  spaces  between  the  faces  of  the 
bricks.  The  top  tier  of  bricks  is  covered  with  the  usual  tie 
and  protecting  course,  and  this  may  be  doubled  on  top  and 
carried  across  from  bench  to  bench.  One  can  readily  see 
that  as  the  bricks  shrink  there  is  some  danger  of  the  gases 
getting  in  between  the  ends  of  the  special  bricks,  but  it  is 
unusual  for  them  to  penetrate  as  far  as  the  faces,  and  for 
stretcher  face  bricks  the  protection  is  ample.  Quoins,  how- 
ever, are  frequently  damaged  on  the  heads,  and  to  protect 
them  the  bricks  are  set  in  triplets  instead  of  doubles.  The 
quoins  are  placed  in  the  middle  tier  and  the  stretchers  in 
the  outer  tiers  on  each  side. 

Roofing  tiles  of  the  porous  type  are  set  in  open  kilns  with- 
out supports  or  stands.  Such  tiles  are  made  of  clays  which 
shrink  very  little  and  which  are  not  liable  to  warpage,  dis- 
tortion and  kiln  marking.  The  floor  of  the  kiln  is  covered 
with  two  or  more  courses  of  bricks  on  edge  to  provide  draft 
space  and  circulation  under  the  mass  of  tiles.  The  tiles  so 
set  are  of  the  interlocking  type,  and  no  matter  how  closely 
they  may  be  nested,  the  projecting  lugs  and  locks  provide 
draft  space.  In  setting,  a bunch  of  tiles  are  nested  as  closely 
as  possible  and  the  number  of  tiles  in  a bunch  is  such  that 
the  thickness  of  the  mass  corresponds  to  the  length. 

The  setting  across  the  kiln  in  each  bench  consists  of  alter- 
nate bunches  of  headers  and  stretchers,  the  tiles  resting  on 
their  edges,  and  are  packed  as  closely  as  possible  and  wedged 
against  the  kiln  walls.  The  second  course  reverses  the  first. 
If  the  first  course  starts  with  a bunch  of  headers  the  second 
course  will  start  with  a stretcher  bunch,  and  the  third  course 
will  duplicate  the  first,  thus  alternating  to  file  top.  A face 
of  set  tiles  will  have  a checker  board  appearance.  Squares, 
or  rectangles,  the  size  of  a tile,  showing  ends  of  the  tiles, 
will  alternate  with  squares  showing  the  side  of  the  tile.  The 
alternation  is  also  carried  out  in  the  benches.  If  the  first 
bench  starts  with  a bunch  of  headers,  the  next  bench  will 
have  stretchers. 

Any  tendency  to  roll  sideways  is  limited  to  a single  header 
bench  of  ten  tiles,  more  or  less,  since  the  stretcher  benches 
prevent  the  extension  of  the  rolling  tendency.  Similarly,  any 


BURNING  CLAY  WARES. 


243 


forward  or  backward  rolling  in  the  stretcher  blocks  is  checked 
by  header  blocks  in  the  adjacent  benches.  It  is  essential 
that  each  course  be  tightly  wedged  against  the  kiln  and  bag 
walls,  and  that  the  tiles  be  in  touch  with  each  other  through- 
out the  mass  of  the  setting.  The  rolling  then  is  limited  to 
the  shrinkage,  which  in  porous  tiles  is  very  slight— frequently 
none  whatever. 

In  some  instances  where  there  is  no  shrinkage,  or  the  kiln 
is  narrow,  it  is  only  necessary  to  put  in  an  occasional  block 
of  stretchers  to  prevent  rolling. 

Vitrified  tiles,  and  such  tiles  as  nest  perfectly — shingle 


tiles  and  Spanish  or  “S”  tiles,  without  lugs  or  locks,  must 
be  set  in  “stands.”  The  stands  are  fire  clay  plates,  2 y2  inches 
thick,  with  the  other  dimensions  corresponding  to  the  width 
and  length  of  the  tiles  usually  around  12  inches  by  16  inches. 
These  plates  are  set  up  to  form  a series  of  pigeon-holes,  or 
“boxes,”  as  shown  in  Fig.  135,  in  which  the  tiles  are  placed. 
The  boxes  are  braced  against  the  kiln  walls,  or  where  this  is 
not  practical,  as  in  some  parts  of  round  kilns,  and  above  the 
bags,  an  occasional  blind  box  is  set  in  the  stand  by  simply 
placing  the  flat  side  of  the  vertical  plate  parallel  with  the 


244 


BURNING  CLAY  WARES. 


face  and  back  of  the  stand,  which  we  also  illustrate  in  Fig. 
135.  The  stands  rest  on  bricks  on  the  kiln  floor,  which  insure 
draft  and  circulation  under  the  stands  and  spacing  the  stands 
several  inches  insures  draft  space  from  top  to  bottom  of 
the  kiln.  The  tiles  are  placed  in  the  boxes  on  the  flat,  on 
end,  or  on  edge,  as  may  be  best  for  any  particular  shape. 
Shingle  tile,  for  instance,  may  be  set  flat,  or  on  edge  in  a solid 
mass  in  stands  of  a suitable  height,  as  shown  in  the  bottom 
tier  in  the  sketch,  but  any  clay  worker  will  appreciate  the 
difficulty  of  burning  to  vitrification  a solid  block  of  clay  with- 
out cracking,  or  checking,  bloating,  or  block  coring.  Such 
solid  setting  is  not  practical  except  the  entire  kiln  is  so  set, 
and  the  needed  time  and  care  given  to  the  burning.  It  would 
be  a serious  waste  of  fuel  and  time  to  burn  a kiln  largely  filled 
with  open  set  ware,  at  the  rate  required  for  a few  stands 
packed  with  tile  in  a solid  mass.  Shingle  tile  may  be  set 
open  by  using  strips  of  clay  to  separate  alternate  pairs  of 
faced  tile.  Several  methods  of  setting  shingle  tile  are  shown 
in  the  sketch. 

Spanish  tiles  are  set  on  end  in  pairs,  closely  nested,  and 
the  pairs  are  separated  by  embedding  them  in  strips  of  clay, 
and  supporting  them  in  a vertical  position  by  similar  strips 
of  clay  on  top.  Interlocking  tile  are  also  embedded  in  strips 
of  clay,  and  are  set  on  edge  or  on  end  as  they  may  best  fit 
the  stands  or  suffer  the  least  deformation  in  burning. 

Finials  and  other  roof  trimmings  are  either  set  in  the 
boxes,  or  on  top  of  the  stands. 

Terra  Cotta  is  also  set  in  stands  called  floors,  but  the  boxes 
are  much  larger  and  the  floors  cannot  be  built  up  in  uniform 
sized  boxes  on  account  of  the  variations  in  the  sizes  of  the 
ware.  The  floor  blocks  must  have  larger  dimensions  in  order 
to  support  the  weight  which  larger  boxes  involve.  There  is 
no  standard  size  for  the  floor  plates  and  posts.  The  floor 
plates  are  commonly  three  inches  thick  and  two  feet  square. 
The  posts  are  thick  walled  hollow  tiles,  about  six  inches 
square  and  vary  in  length  up  to  thirty  inches.  Low  floors  may 
use  short  posts,  or  lacking  these  one  or  more  posts  may  be 
placed  on  edge  and  extra  high  floors  may  have  the  posts 
capped  by  one  on  edge.  The  aim,  however,  is  to  have  the 
structure  as  open  as  possible  to  get  the  full  benefit  of  the 
radiation  from  the  walls.  Manufacturers  differ  in  regard  to 
the  best  kiln  height  for  terra  cotta.  The  older  and  more  com- 
mon kiln  is  15  to  18  feet  high,  and  there  may  be  five  floors. 
This  involves  considerable  work  in  building  the  floors,  set- 


BURNING  CLAY  WARES. 


245 


ting  and  drawing  the  ware  and  taking  down  the  floors.  Others 
advise  low  kilns,  even  as  low  as  merely  head  room  for  the 
workmen,  and  limiting  the  floors  to  one,  or  two  at  most. 
Economy  in  heat  is  claimed  for  the  high  kilns  and  economy 
in  labor  for  the  low  kilns. 

Sewer  pipes  are  set  with  the  spigot  end  down  and  the 
large  sizes  are  placed  on  rings  of  green  ware  to  take  up  the 
shrinkage  strain.  The  setting  is  in  a series  of  circles  on 
account  of  the  use  of  the  crane  for  handling  large  sizes.  As 
has  been  previously  described,  the  spaces  between  the  bags 
are  filled  with  small  sizes,  three  lengths  high  or  sometimes 
four.  Then  come  rings  of  intermediate  sizes,  and  inside  these 
are  the  large  pipes,  which  usually  may  not  be  set  next  to 
the  bags  where  they  would  be  within  the  variable  influence 
of  the  high  temperature  gases  leaving  the  bags,  resulting  in 
serious  loss.  The  setting  height  is  usually  four  standard 
lengths,  or  equivalent  in  longer  lengths.  The  setting  is  such 
that  the  sockets  are  in  touch,  which,  because  of  the  circular 
setting,  is  a sufficient  brace,  especially  in  view  of  the  stability 
of  the  columns  of  large  pipe.  Smaller  sizes  are  set  inside 
the  larger  sizes — “stuffing” — but  there  must  be  sufficient  dif- 
ference in  size  to  insure  a clear  annular  space  between  the 
two,  otherwise  the  inside  of  one  and  the  outside  of  the  other 
will  not  have  a good  glaze. 

Elbows,  branches,  tees,  traps,  etc.,  are  largely  set  on  top. 
Tees  and  branches  may  be  set  in  the  regular  columns,  but 
elbows,  traps,  etc.,  must  be  set  on  top.  Some  of  these  shapes 
may  be  set  in  the  top  course  without  any  difficulty,  the  spigot 
resting  in  the  socket  of  the  standard  pipe  below.  Elbows,  if 
set  singly  with  the  spigot  end  down,  would  be  top  heavy  on 
account  of  the  socket  and  would  be  liable  to  topple.  We  have 
seen  them  set  in  this  way  and  the  two  braced  against  each 
other  with  a clot  of  clay  to  keep  them  apart  for  draft  space. 
The  safest  setting  for  elbows  is  to  use  a ring  of  clay  in  the 
socket  of  the  lower  pipe,  the  socket  of  the  elbow  resting  on 
the  projecting  ring,  and  the  elbow  may  be  tipped  at  an  angle 
to  equalize  the  weight  on  the  supporting  column,  at  the  same 
time  insuring  a more  direct  course  for  the  salt  fumes.  In 
various  simple  ways  the  top  of  the  kiln  may  be  set  with  spe- 
cials, often  without  any  bracing,  while  unstable  pieces  may 
be  braced,  one  against  another,  by  clots  and  strips  of  clay. 

Enameled  bricks  are  occasionally  set  in  stands  or  in  sag- 
gers, especially  where  the  enamel  has  a tendency  to  run  and 


246 


BURNING  CLAY  WARES. 


form  a thick  edge  very  noticeable  in  the  wall  in  certain 
angles  of  reflected  light.  In  stands  or  saggers  they  may  be 
set  with  the  enamel  edges  up  and  the  enamel  spreads  evenly. 

Silica  bricks  were  formerly  set  in  the  usual  checker  fashion 
and  this  method  may  still  be  used  in  a number  of  yards.  In 
several  yards  a bench  type  of  setting  shown  in  Fig.  136  is  the 
modern  practice.  A course  of  burned  bricks  is  set  on  the 
floor  in  four  brick  benches  the  full  width  of  the  kiln,  and  these 
bricks  are  spaced  to  permit  the  escape  of  the  kiln  gases  to 


Figure  136.  Figure  137. 

Silica  Brick  Setting.  Silica  and  Magnesite  Brick  Setting. 

the  kiln  floor  outlets.  This  course  of  bricks  is  covered  with  a 
flat  course  of  burned  bricks  laid  tight,  and  upon  this  floor 
begins  the  setting  of  the  green  ware.  The  bricks  are  set 
headers  throughout  except  at  the  ends  of  the  benches  where 
stretchers  are  necessary  in  order  not  to  rack  back  except  as 
desired.  The  setting  is  carried  up  to  a height  of  about  eigh- 
teen courses  and  covered  with  a flat,  tight  course  of  burned 
bricks.  On  this  upper  floor  are  usually  set  the  shapes, 
blocks,  etc.,  interspersed  with  standard  bricks.  The  top  is 


BURNING  CLAY  WARES. 


247 


finished  with  a single,  or  several  courses  of  standards  on 
edge.  The  setting  is  much  higher  than  the  usual  face  and 
paving  brick  setting,  being  around  thirty-six  courses  on  edge, 
where  the  latter  vary  between  twenty-four  and  thirty  courses. 

The  four  brick  benches  are  spaced  not  more  than  three 
inches  and  these  spaces  are  the  draft  flues  from  top  to  bot- 
tom of  the  kiln.  It  is  seen  from  the  setting  that  there  can  be 
no  draft  down  through  the  mass  of  ware,  but  each  bench  is 
completely  surrounded  by  the  kiln  gases,  and  the  ware  within 
the  bench  is  burned  chiefly  by  conduction  and  radiation,  and 
by  convection  only  to  whatever  extent  the  gases  are  baffled 
back  and  forth  by  the  varying  gas  pressure  in  the  draft 
spaces. 

The  setting  is  perhaps  a consequence  of  the  burning  which 
differs  materially  from  the  ordinary  burning  process.  The 
method  followed  is  to  use  deep  furnaces  with  free  egress 
into  the  kiln,  and  to  heavily  charge  the  furnaces  with  fuel 
each  firing  period.  In  consequence  the  kiln  and  flue  system 
even  to  the  top  of  the  stack  are  filled  with  unburned  gas. 
The  purpose  is  to  burn  the  gas  in  contact  with  the  ware  from 
top  to  bottom  of  the  kiln,  thus  getting  flame  temperature.  At 
least  during  some  stage  between  the  firing  periods  there  is 
perfect  combustion  conditions  and  maximum  temperatures 
and  the  setting  is  practically  such  as  to  quickly  develop  the 
temperature  in  the  bottom  of  the  kiln. 

Magnesite  bricks  are  set  with  silica  bricks.  Where  both 
products  are  made  in  the  same  factory  we  hare  seen  the 
magnesite  bricks  set  with  silica  bricks  header  and  stretcher 
in  the  usual  checker  fashion,  except  that  the  header  courses 
were  alternate  silica  and  magnesite,  and  the  stretchers  were 
all  silica.  The  setting  was  such  that  the  stretchers  broke 
joints  on  the  silica  headers.  The  silica  bricks  being  slightly 
larger  than  the  magnesite  and  expanding  in  the  heating  up, 
the  magnesite  bricks  carried  no  weight.  In  other  words,  the 
silica  bricks  were  set  to  form  a series  of  pigeon  holes  or 
boxes  in  alternate  courses,  and  in  these  boxes  were  placed 
the  magnesite  bricks — one  in  each  space  spanned  by  the 
silica  stretchers. 

Some  factories  make  only  magnesite  bricks,  and  these  are 
set  in  burned  silica  brick  stands.  One  method  of  such  set- 
ting is  shown  in  Fig.  137.  The  boxes  are  built  up  of  silica 
bricks  and  covered  tightly  with  flat  courses  of  silica  bricks,  in 
four  brick  benches.  Each  box  is  filled  with  magnesite  bricks 


248 


BURNING  CLAY  WARES. 


placed  on  end.  In  the  upper  part  of  the  kiln  there  are  set 
all  magnesite  bricks  to  whatever  height  they  will  stand, 
usually  six  to  eight  courses,  in  alternate  finger  spaced  and 
tight  courses. 

By  such  setting  the  kiln  content  is  about  40  per  cent, 
magnesite. 

The  burning  is  the  same  as  that  of  silica  bricks  and  the 
temperatures  required  are  around  cone  20 — 2780  degrees. 

Pottery,  porcelain,  abrasives,  etc.,  are  burned  in  saggers 
and  the  problems  involved  are  those  of  filling  the  saggers, 
supporting  the  ware,  etc.,  and  are  beyond  the  province  of 
these  articles.  The  filled  saggers  are  set  in  the  kilns  ill 
“bungs,”  and  the  chief  difficulty  is  to  place  the  “bungs”  in  the 
proper  position  in  the  kiln  to  get  the  desired  temperature  for 
the  ware  enclosed. 

The  setting  in  a ring  kiln  and  in  a direct  coal-fired  cham- 
bered kiln  require  some  knowledge  of  the  operation  of  such 
kilns,  particularly  the  ring  kiln. 

The  draft  in  a ring  kiln  is  largely  horizontal,  and  unless 
the  fires  advance  properly  there  is  danger  of  losing  them, 
and  greatly  delaying  the  burning  of  the  ware,  or  of  getting 
unsatisfactory  results.  The  heat  should  be  kept  in  advance 
in  the  bottom  of  the  kiln,  which  is  difficult  to  do  if  the  top  of 
the  kiln  is  set  very  open,  or  if  there  is  too  much  free  area  over 
the  top  of  the  ware.  The  tendency  in  such  instances  is  that 
the  gases  from  the  trace  flues  will  rise  vertically  from  the 
coal  and  then  turn  forward  in  the  upper  part  of  the  kiln, 
completing  the  combustion  there,  and  thus  finishing  the  ware 
in  the  upper  part  of  the  kiln  before  the  bottom  ware  is 
finished.  We  then  get  the  top  burned  first,  and  have  a maxi- 
mum temperature  on  top  in  advance  of  the  fire  below. 

If  the  setting  is  relatively  close  on  top,  the  air  and  gases 
are  forced  to  follow  the  trace  flues,  thus  keeping  the  heat  in 
advance  on  the  floor  of  the  kiln,  and  the  upper  ware  may  be 
finished  by  radiation,  conduction  and  convection,  and  keep  in 
pace  with  the  bottom  ware. 

A ring  kiln  constructed  with  drop  arches  at  intervals  of 
about  twelve  feet  has  a decided  advantage,  in  that  the  arch 
acts  as  a damper  to  retard  the  flow  of  the  gases  in  the  open 
space  between  the  ware  and  the  crown  of  the  kiln,  and  forces 
them  down  among  the  ware.  Sometimes  a dead  wall  of  green 
ware  is  set  in  conjunction  with  the  drop  arch,  thus  actually 
dividing  the  tunnel  into  separate  compartments,  connected. 


BURNING  CLAY  WARES. 


249 


one  with  the  other  by  the  trace  flues.  This  gives  a fresh 
start  in  every  section,  and  it  is  impossible  for  the  top  to  get 
much  in  advance  of  the  fires  in  the  trace  flues.  To  the  con- 
trary, it  may  be  necessary  to  set  the  ware  to  collect  a por- 
tion of  the  fuel  at  different  levels  in  order  to  get  a uniform 
burn.  The  bricks  forming  the  vertical  firing  flues  are  stag- 
gered in  such  a way  as  to  collect  the  fuel  on  the  edge  of  the 
projecting  bricks,  and  as  it  spills  it  is  caught  by  lower 
courses  of  bricks  step  by  step  to  the  lower  part  of  the  kiln, 
thus  spreading  it  out  in  a fan  or  inverted  “V”  shape,  prac- 
tically covering  the  width  of  the  kiln.  Sometimes  loose  bricks 
are  set  in  the  vertical  flues,  which  at  any  stage  in  the  firing 
may  be  turned  flat  to  provide  a shelf  for  the  collection  of 
fuel  at  any  level  or  series  of  levels. 

Otto  Bock  and  Ernst  Schnatolla,  and  other  German  engi 
neers,  describe  and  illustrate  methods  of  setting  and  kiln  con- 
struction which  give,  in  a ring  kiln,  firing  compartment  sep- 
arate from  the  ware  and  in  some  instances  convert  the  kiln 
into  separate  compartments.  American  clayworkers  are  not 
much  interested  in  these  changes.  Some  of  the  changes  in- 
volve a lot  of  dead  work  which  would  be  prohibitive  in  Amer- 
ican factories,  while  others  are  adapted  to  small  kilns  only, 
which  are  much  more  common  in  Europe  than  in  America. 

American  practice  requires  large  kilns  and  a maximum 
capacity  of  some  kind  of  ware,  and  we  cannot  afford  to  muss 
up  our  kilns  with  a lot  of  equipment  or  a variety  of  wares  not 
essential  in  other  types  of  kilns.  Instead  we  scrap  the  kiln 
and  build  the  better  type. 


250 


BURNING  CLAY  WARES. 


CHAPTER  X. 

THE  CONTINUOUS  KILN. 

IT  IS  a question  whether  we  should  not  adopt  a more  dis- 
tinctive name.  The  common  up-draft  kilns  as  operated  in 
the  Hudson  River  and  Chicago  districts  are  continuous  in 
their  operation  and  are  often  called  continuous  kilns.  The 
setting,  followed  by  the  burning,  cooling  and  drawing,  pro- 
gresses in  stages  from  end  to  end  of  the  kiln  shed,  a prac- 
tically continuous  operation,  though  not  in  the  sense  as  we 
apply  it.  The  designation  “continuous”  is  used,  however,  and 
leads  to  some  confusion. 

The  connection  and  continued  operation  of  a number  of 
periodic  kilns  in  order  to  make  use  of  the  waste  heat  is  just 
as  much  continuous  as  the  operation  of  a battery  of  compart- 
ments. Such  a method  of  burning  is  usually  termed  a “Sys- 
tem” and  bears  the  name  of  its  promoter. 

We  designate  a single  battery  of  compartments,  as  semi- 
continuous,  in  consequence  of  the  independence  of  successive 
burns.  Each  burn  starts  at  one  end  of  the  series  of  com- 
partments and  is  completed  at  the  other  end.  Subsequent 
burns  are  merely  repetitions  and  are  entirely  independent  of 
th  preceding  operation.  The  operation  of  the  kiln  as  a whole 
is  periodic,  but  the  several  compartments  are  in  series,  and 
the  progress  of  the  fires  during  each  burn  is  continuous,  as  we 
use  the  term.  If  we  introduce  a return  heating  flue  the  semi- 
continuous  kiln  becomes  fully  continuous  in  its  operation,  but 
its  designation  is  the  same. 

If  we  build  a duplicate  series  of  compartments  for  the  re- 
turn, the  double  series  becomes  a continuous  kiln,  or  in  other 
words,  two  semi-continuous  kilns  are  a continuous  kiln. 

The  term  continuous  applies  to  the  method  of  operation, 
hut  instead  we  should  have  a name  descriptive  of  the  prin- 
ciple involved. 

Continuous  kilns  are  sometimes  called  regenerative,  and 
this  is  a better  term,  in  that  it  refers  to  the  principle.  Strictly 


BURNING  CLAY  WARES. 


251 


speaking,  there  is  no  regeneration  of  the  gases,  no  renewal  of 
the  heat  value,  but  the  term  regenerative  has  become  firmly 
fixed  in  metallurgical  operations,  and  means  heating  up  the 
incoming  gases  by  the  waste  heat  from  the  hearth,  which  is 
identically  the  same  principle  as  our  continuous  kilns. 

Economizer,  which  in  steam  and  combustion  engineering 
means  recovering  for  use  in  the  boiler  or  hearth  the  other- 
wise waste  “heat  from  it,  is  a better  term  than  either  continu- 
ous or  regenerative.  Regenerator  in  metallurgy  is  identical 
with  economizer  in  steam  engineering,  but  the  latter  is  the 
most  consistent. 

The  continuous  up-draft  kiln  operation  mentioned  above 
could  not  legitimately  be  called  a regenerator  or  economizer 
operation. 

The  semi-continuous  and  continuous  kilns  are  truly  econo- 
mizers. 

The  periodic  down-draft  kilns  in  series  are  regenerative, 
but  we  would  not  designate  them  as  a kiln,  instead  and 
X»roperly,  when  adapted  to  the  utilization  of  their  own  waste 
heat,  they  become  an  economizer  system. 

There  are  many  economizer  kilns  in  use  and  in  prospect 
in  this  country.  It  is  not  our  purpose  merely  to  describe 
them.  Rather  we  wish  to  show  the  development  of  the  prin- 
ciples and  the  chief  modifications.  There  are  a number  of 
excellent  kilns  which  we  will  not  mention  simply  because  they 
differ  slightly  from  others  herein  presented. 

We  have  in  mind  that  the  presentation  of  principles  may 
be  of  some  assistance  to  economizer  kiln  operators  in  improv- 
ing their  kilns.  The  study  of  the  development  of  kilns  is 
interesting  and  one  can  frequently  trace  the  idea  back  to 
other  simple  types  of  kilns  and  improvements  are  often  com- 
bination of  existing  types.  This  leads  to  progress  in  the  devel- 
opment to  which  purpose  this  presentation  is  dedicated. 

Economizer  Kilns  in  General. 

The  great  factor  in  favor  of  the  economizer  kilns  is  the 
saving  of  fuel.  Fifty  per  cent,  saving  is  a conservative  esti- 
mate, and  many  operations  will  show  60  to  70  per  cent. 

The  development  of  these  kilns  in  this  country  was  slow 
for  the  following  reasons : 

1)  American  coal  is  high  grade,  and  in  the  past  cheap. 
Any  requisite  temperature  can  be  obtained  from  our  coals  by 
direct  cold  air  combustion  and  there  is  no  need  to  adopt  econ- 
omizer kilns  to  get  temperatures  from  low  grade  fuel. 


252 


BURNING  CLAY  WARES. 


(2)  The  early  kilns  were  small — in  general  having  a 
weekly  capacity  scarcely  equal  to  the  daily  output  of  an  Amer- 
ican factory. 

(3)  High  priced  American  labor.  It  is  frequently  claimed 
that  the  labor  cost  of  operating  an  economizer  kiln  is  less 
than  that  of  a battery  of  periodic  kiln  of  the  same  capacity. 
This  claim  can  be  substantiated  only  in  isolated  instances,  and 
certainly  not  in  the  operation  of  the  small  early  kilns.  As 
the  capacities  of  the  conomizer  kilns  increase  from  5,000  to 

20.000  bricks  per  day  up  to  present  capacities  in  excess  of 

100.000  bricks  per  day,  there  will  undoubtedly  be  some  econ- 
omy in  labor,  but  taking  the  average  over  the  country,  past 
and  present,  the  records  will  show  no  economy  in  labor,  and 
in  many  instances  an  increased  labor  cost  sometimes  offsetting 
the  saving  in  fuel,  though  on  an  average  the  net  balance  is  in 
favor  of  the  economizer  kiln.  However,  labor  cost  had  its 
effect  in  retarding  the  growth  of  the  kiln. 

(4)  Unsatisfactory  operation  of  early  kilns.  Our  first 
kilns  were  of  foreign  design  and  we  simply  enlarged  them  to 
meet  our  needs  and  thereby  got  into  difficulties.  Moreover,  the 
small  foreign  kilns  burned  two  and  even  three  kinds  of  ware 
to  accommodate  the  variable  temperatures,  whereas  the  Amer- 
ican output  in  the  enlarged  kilns  was  of  one  kind  only. 

(5)  America  demands  a better  quality  of  ware  than  that 
in  foreign  countries.  Our  facades  are  a surprise  to  visiting 
engineers.  Many  of  these  effects  were  impossible  in  the  earlier 
kilns,  and  some  of  them  are  impractical  in  the  modern  kiln, 
or  at  least  the  modern  kiln  has  not  yet  proven  its  adaptability. 
Besides  this  there  were  excessive,  scumming  difficulties,  and  a 
swelling  of  the  product  due  to  bloating  in  consequence  of  lack 
of  oxidization,  also  streaked  edges  in  certain  line  of  face 
bricks.  These  troubles  are  attributed  to  sulphur  gases,  the 
sulphur  not  only  coming  from  the  coal,  but  often  in  greater 
quantity  from  the  ware  itself,  thus  developing  a much  larger 
volume  of  sulphur  gas  than  in  a down-draft  kiln. 

Limey  clays,  which  ordinarily  burn  a pale  red  to  greenish 
buff,  and  in  mass  altogether  unsightly.  The  lime-iron-silicate 
red,  sometimes  buff,  with  all  intermediate  shades  irregularly 
intermingled,  in  fact  individual  bricks  are  streaked  red  and 
buff,  and  in  mass  altogether  unsightly.  The  lime-iron-silicate 
buff  color  is  materially  aided  in  its  development  by  reducing 
kiln  conditions,  which  were  easily  obtainable  and  during  some 
stages  of  the  firing  normally  prevailed  in  down-draft  kilns, 


BURNING  CLAY  WARES. 


253 


while  in  the  economizer  kilns  the  large  excess  of  air  main- 
tained unfavorable  oxidizing  conditions.  Seger  explained  the 
cause  of  this  behavior  and  how  to  correct  it. 

(6)  The  cost  of  the  kiln.  Many  maunfacturers  balked  at 
the  high  cost  of  an  initial  installation,  although  piece-meal 
they  invested  a larger  sum  in  periodic  kilns. 

While  the  development  in  this  country  has  been  slow,  yet 
it  has  gone  steadily  forward  and  today  the  kiln  occupies  a 
prominent  place  in  our  industries.  The  capacities  have  been 
enlarged  to  meet  our  requirements  and  the  results  have  been 
improved  until  in  several  lines  of  common  wares  they  equal 
those  of  the  down-draft  kiln.  In  consequence  of  this  advance 
the  economizer  kiln  has  largely  entered  the  field  of  the  down- 
draft  kiln. 

In  common  bricks,  which  were  largely  burned  in  up-draft 
kilns,  the  desire  for  a longer  campaign,  the  demand  for  a 
better  product,  the  need  for  reduction  in  cost,  however  small 
it  might  be,  to  meet  competition,  and  the  necessity  of  elimi- 
nating the  smoke  and  gas  nuisance,  especially  in  city  districts, 
have  led  to  the  replacement  of  up-draft  kilns  by  economizer 
kilns. 

Face  brick  have  taken  up  the  kiln  in  some  degree,  but  not 
extensively  because  of  the  difficulty  in  producing  the  varied 
color  effcts  which  the  trade  demands. 

Salt  glazing  has  not  been  successfully  done  in  the  econo- 
mizer kilns,  although  one  or  two  kilns  have  been  built  for 
this  product,  the  results  have  been  unsatisfactory,  and  other 
lines  of  ware  had  to  be  developed  to  keep  the  kilns  in  opera- 
tion. Several  Mendheim  kilns  in  Germany  are  said  to  be  pro- 
ducing salt  glazed  ware,  but  this  kiln  in  the  type  used  for  salt 
glazing  is  in  a measure  a periodic  down-draft  kiln  and  it  does 
not  fully  use  the  economizer  principle.  We  are  informed  that 
when  the  salt  glazing  period  is  reached  in  any  compartment 
it  is  disconnected,  salted  and  shut  off  and  so  remains  until 
cool  and  emptied.  Years  ago  the  writer  experimented  with 
an  economizer  kiln  in  salt  glazing  and  had  some  excellent 
results,  but  the  operation  as  a whole  was  not  satisfactory  and 
the  work  discontinued.  It  seemed,  however,  to  be  possible, 
and  it  is  strange  that  in  the  intervening  twenty-five  years  the 
problem  has  not  been  solved.  It  requires  a kiln  so  designed 
that  the  compartment  being  salted  can  be  cut  out  of  the  series 
during  the  salting  period  and  afterwards  until  the  salt  fumes 
have  been  driven  off.  We  have  seen  plans  of  a kiln  in  which 


254 


BURNING  CLAY  WARES. 


such  operation  is  possible,  but  we  do  not  know  of  any  kiln 
having  been  built. 

The  fire  brick  industry  was  the  first  to  take  up  the  econo- 
mizer kiln  in  this  country  and  the  last  to  make  any  extensive 
use  of  it.  A gas  fired  tunnel  kiln  was  built  in  Maryland  more 
than  thirty  years  ago,  and  also  a small  kiln  was  built  in 
Illinois.  These  were  the  first  so  far  as  we  know,  and  were 
in  use  until  within  the  last  year  or  two.  The  third  was  built 
in  Ohio  in  1884-5  for  face  bricks,  but  the  product  was  changed 
to  fire  bricks  and  later  came  back  to  face  bricks,  although 
they  were  made  from  fire  clay  and  differed  from  fire  bricks 
only  in  size.  This  kiln  is  no  longer  in  use.  Two  gas-fired 
kilns  were  built  in  Pennsylvania  in  1891  and  one  in  Ohio  the 
same  year  for  fire  bricks,  but  all  were  abandoned  after  a 
short  trial.  About  this  time,  or  a year  or  two  later,  two 
economizer  kilns  were  built  in  Missouri  for  fire  bricks,  but 
neither  are  in  operation  today.  There  are  a number  of  econo- 
mizer kilns  burning  other  products  made  from  fire  clays  and 
which  are  used  in  burning  fire  bricks  when  the  demand 
justifies,  but  the  fire  brick  companies  making  fire  bricks  ex- 
clusively have  not  taken  kindly  to  the  economizer  kiln.  It  is 
inexplicable  that  the  industry  which  first  took  up  the  economizer 
kiln  in  this  country  and  for  which  such  a kiln  is  particularly 
fitted  because  of  the  possible  high  temperature  from  economizer 
operation,  should  not  have  successfully  developed  it,  especially 
in  view  of  the  fact  that  refractory  ware  is  less  exacting  in  its 
temperature  range  than  other  lines  of  ware  for  which  the 
economizer  kiln  has  proven  successful. 

Recently  the  fire  brick  industry  has  taken  up  the  car  tunnel 
kiln  and  there  are  four  or  five  installations  which  are  running 
successfully. 

Pottery  manufacturers  have  taken  little  or  no  interest  in 
the  economizer  kilns  until  recently  they  have  taken  up  the  car 
tunnel  kiln  and  are  leading  representatives  in  the  use  of  this 
kiln. 

The  terra-cotta  field  was  not  invaded  by  the  economizer 
kiln  except  in  a single  instance  in  1891,  and  the  kiln  in  ques- 
tion was  shortly  abandoned. 

Both  the  ring  kiln  and  the  compartmenet  kilns  are  exten- 
sively used  in  the  manufacture  of  common  bricks,  face  bricks, 
paving  bricks  and  hollow  ware.  The  compartment  finds  a 
wider  field  in  face  bricks,  but  since  the  higher  product  includes 
the  lower,  it  is  equally  efficient  in  the  lower  grade  wares. 


BURNING  CLAY  WARES. 


255 


The  determination  of  the  type  of  kiln,  tunnel  or  compart- 
ment for  any  product  is  a question  which  requires  considera- 
tion. The  advocates  of  the  car  tunnel  kiln  voice  the  opinion 
that  it  will  displace  the  simple  tunnel  and  compartment  types, 
but  this  is  inconceivable. 

The  compartment  kiln  is  a more  advanced  type  than  the 
tunnel  kiln  and  it  will  have  the  preference  in  higher  grade 
wares.  In  its  simplest  form  the  chief  advantage  is  a greater 
degree  of  down-draft,  which  is  better  than  horizontal  draft. 
In  the  more  advanced  types  the  draft  is  fully  down,  and  the 
firing  is  not  in  contact  with  the  ware.  We  thus  get  a better 
heat  distribution  and  a cleaner  product.  In  fact,  the  double 
gas  compartment  kilns  are  identically  rectangular  down-draft 
kilns  with  the  economy  of  heat  recuperation.  The  setting  is 
uniform  throughout  and  there  are  no  trace  or  vertical  flues 
in  the  set  ware.  Moreover,  the  compartments  are  large,  per- 
mitting the  use  of  a double  track  within  the  kiln  for  setting 
wrork,  and  the  wicket  work  is  reduced  to  the  minimum. 

In  a tunnel  kiln,  turntables  or  transfer  cars  must  be  used 
inside  the  kiln  and  only  one  car  at  the  setting  face  is  prac- 
tical. This  must  be  unloaded  and  removed  before  a second 
car  can  be  placed,  thus  retarding  the  rate  of  setting.  The 
labor  per  ton  of  ware  is  slightly  greater  in  the  operation  of 
the  tunnel  kiln  than  is  in  the  compartment  kiln. 

The  compartment  kiln,  on  the  other  hand,  costs  more  than 
the  tunnel  kiln,  and  its  maintenance  is  greater,  which  offsets 
more  or  less  the  greater  labor  cost  in  the  tunnel  kiln. 

The  tunnel  kiln  probably  leads  in  the  number  of  kilns  in 
use  in  this  country,  but  so  close  do  they  run  together  that 
one  is  not  justified  in  making  a positive  statement.  Each  kiln 
has  its  field  of  work  and,  though  the  fields  overlap,  yet  they 
are  not  identical.  The  tunnel  kiln  is  used  in  the  manufacture 
of  common  bricks,  fire-proofing  extensively,  paving  blocks  and 
drain  tile.  The  latter  product  because  of  the  setting  diffi- 
culty is  preferably  burned  in  a compartment  kiln.  The  com- 
partment kiln  is  used  for  the  above  mentioned  products,  and 
in  addition  is  predominant  in  face  bricks  and  roofing  tile. 

The  several  types  are  now  firmly  established  in  this  coun- 
try and  the  high  price  of  coal  during  the  war  has  induced 
manufacturers  of  clay  ware  to  study  the  kilns  and  give  them 
much  greater  attention  than  in  the  past.  It  is  likely  that 
there  will  be  some  recession  in  fuel  costs,  but  the  price  will 
never  return  to  that  before  the  war,  and  the  result  will  be 
much  more  rapid  advance  in  the  use  of  economizer  kilns. 


256 


BURNING  CLAY  WARES. 


Kiln  Dampers. 

The  damper  in  an  economizer  kiln  has  been  a perplexing 
problem.  The  relative  vacuity  greatly  increases  the  leakage 
which  materially  interferes  with  the  operation  of  the  kiln. 
In  the  older  types  of  kilns  and  in  some  modern  kilns,  the 
dampers  are  seated,  and  the  flues  being  within  the  kiln  walls 
and  frequently  subjected  to  a red  heat,  the  deterioration  of 
the  dampers  is  rapid  and  their  effectiveness  materially  nullified. 

We  have  used  flanged  circular  covers  with  an  annular 
flanged  seat,  sealing  with  sand,  but  the  covers  would  bulge, 
the  frames  warp,  and  the  sand  be  carried  away  by  the  strong 
draft. 

Fire  clay  blocks  on  a flat  seat  were  better,  but  were  subject 
to  frequent  breakage  and  were  difficult  to  replace.  Cast  iron 
dampers  in  a flat  seat  were  still  better  chiefly,  however,  in 
that  the  construction  was  such  that  the  dampers  could  be 
readily  replaced,  and  this  we  consider  an  important  factor  in 
any  kind  of  a permanent  damper. 

A sliding  damper  is  practically  worthless  in  an  economizer 
kiln,  with  the  possible  exception  of  a heavy  fire  clay  block,  or 
blocks  in  a cast  iron  frame,  set  at  such  an  angle  that  the 
weight  of  the  damper  will  hold  it  firmly  on  the  seat. 

A widely  used  damper  is  a conical  valve  in  a seat  adapted 
to  it.  The  seat  may  be  a fire  clay  block  or  cast  iron,  and  the 
valve  is  usually  cast  iron.  This  type  of  damper  holds  its 
shape  fairly  well,  but  at  best  it  is  far  from  satisfactory. 

The  hood  or  goose-neck  is  the  only  satisfactory  connec- 
tion between  kiln  and  flues.  When  it  is  removed  and  the 
openings  covered  and  sealed,  the  disconnection  is  absolute. 
There  may  be  some  leakage  around  the  covers,  but  they  are 
fully  exposed  on  the  kiln  top  or  on  the  ground  and  can  be 
readily  inspected  and  tested.  The  use  of  hoods  has  solved 
another  problem.  The  flue  system  in  a modern  kiln  is  com- 
plicated and  there  are  frequently  cross-overs.  Ordinarily,  in 
making  such  cross-overs,  one  set  of  flues  must  be  under  or 
over  the  other,  but  with  hoods  for  the  connections  the  flues 
may  usually  be  on  the  same  level. 

It  frequently  happens  that  one  flue  serves  two  or  more 
purposes,  for  instance,  to  supply  gas  during  the  burning,  for 
hot  air  during  the  cooling,  and  perhaps  for  draft  in  the  ad- 
vanced compartments.  The  hood  or  goose-neck  will  make 
each  of  these  connections  without  any  possibility  of  inter- 
ference from  the  other  two. 


BURNING  CLAY  WARES. 


257 


Open-top  Economizer  Kilns. 

The  open-top  kiln  has  been  illustrated  and  described  under 
the  head  of  up-draft  kilns. 

Ring  or  Tunnel  Kilns. 

Efforts  to  develop  an  economizer  operation  during  the  lat- 
ter part  of  the  eighteenth  century  and  the  early  years  of  the 
nineteenth  century  are  recorded,  but  the  first  kiln  having  the 
basic  principle  of  the  ring  kiln  was  patented  by  Arnold  in 
1839,  and  although  a failure,  yet  to  it  may  be  traced  the  cause 
of  the  nullification  of  Hoffman’s  later  patent. 

Arnold  used  a horizontal  transverse  flue  for  each  section 
in  the  bottom  of,  or  under,  the  ware,  in  which  the  firing  was 
done,  and  the  position  of  this  flue  was  fixed.  The  gases  rising 
from  this  combustion  flue  passed  forward  through  heating  and 
watersmoking  sections  until  the  heat  value  was  practically 
exhausted,  when  they  were  drawn  off  into  the  stack.  The 
combustion  air  entered  through  the  open  sections  and  advanced 
through  the  cooling  sections  to  the  burning  section. 

The  Hoffman  kiln  had  no  fixed  combustion  flue  or  shaft, 
but  instead,  vertical  firing  shafts  were  provided  in  setting  the 
ware,  as  in  the  tunnel  kilns  now  in  use. 

The  Hullman  kiln,  patened  in  1854,  was  distinctly  annular 
since  it  has  the  annular  tunnel  enclosing  an  annular  smoke 
flue  with  the  smoke  stack  in  the  center. 

The  Hoffman  kiln  was  patented  in  1858  by  Friedrich  Hoff- 
man and  A.  Licht,  and  though  the  patent  was  twice  extended, 
it  was  finally  abrogated  in  1870. 

The  credit  for  the  ring  or  tunnel  kiln,  known  the  world  over 
as  the  Hoffman  type  of  economizer,  belongs  to  Hoffman  be- 
cause he  made  the  kiln  a success,  whether  he  originated  the 
basic  idea  or  not. 

The  annular  plan  has  been  replaced  by  the  rectangular 
arrangement  of  the  tunnels  either  as  a single  construction 
with  a division  wall  separating  the  tunnels  or  as  separate 
parallel  tunnels  connected  at  the  ends  by  circular  tunnels  (a 
flattened  ellipse)  or  simple  cross-over  flues. 

Fig.  138  is  a cross  section  and  Fig.  139  a plan  of  a Hoffman 
type  of  an  economizer  kiln.  The  kiln  illustrated  is  a marked 
advance  over  the  original  Hoffman  kiln,  or  any  kiln  designed 
by  Hoffman.  There  have  been  many  modifications  of  and  im- 
provements on  the  original  kiln.  As  previously  mentioned, 
the  original  kiln  had  a circular  tunnel,  with  included  smoke 
flue  and  central  stack.  This  gave  way  to  the  oblong  and  rec- 


258 


BURNING  CLAY  WARES. 


tangular  arrangement  of  the  tunnel,  and  finally  to  separate 
tunnels — independent  single  kilns — connected  at  the  ends,  thus 
combining  them  into  a fully  continuous  kiln.  The  earlier 
rectangular  kilns,  and  some  of  the  modern  ones,  have  the 
draft  flue  in  the  longitudinal  division  wall,  with  inlets  from 
the  tunnel  at  the  floor  level,  connecting  with  the  main  draft 
flue  by  uptakes,  and  the  draft  is  controlled  by  dampers  op- 
erated from  the  top  of  the  kiln,  as  shown  in  dotted  lines  in 
Fig.  138. 

The  stack,  as  stated,  in  the  circular  kilns  was  in  the  center, 
and  in  the  development  of  the  oblong  kiln  it  was  natural  to 
place  the  stack  somewhere  in  the  center  wall  between  the 
parallel  tunnels.  If  the  stack  draft  is  to  be  used,  the  stack 
should  be  included  within  the  kiln  walls,  because  we  carry  the 
gases  through  the  ware  until  they  have  given  up  their  heat 
and  become  fully  saturated  with  moisture.  The  operation  of 


Figure  138. 


the  kiln  depends  upon  the  buoyancy  of  the  gases  leaving  the 
kiln,  which  constitutes  the  draft,- and  where  the  stack  is  within 
the  kiln  walls,  its  base  is  kept  hot  by  conduction  and  the  draft 
is  stronger  in  consequence.  The  importance  of  this  is  evident 
from  the  previous  discussion  of  stacks.  In  some  kilns  the 
draft  is  from  the  top  by  using  a hood  (goose-neck)  over  the 
feed  holes  on  top  of  the  kiln  or  over  special  draft  holes  through 
the  crown  and  extending  the  hood  to  cover  a corresponding 
opening  through  the  top  pavement  of  the  kiln  into  the  central 
draft  flue. 

Where  the  stack  (or  fan)  is  outside  the  kiln  walls,  a down 
take  flue  draws  the  gases  into  a transverse  flue  under  the  kiln 
floor  and  thence  to  the  stack  or  fan. 

In  the  above  described  draft  arrangement  the  draft  con- 
nections are  through  the  inner  wall,  which  has  the  effect  of 
drawing  the  heat  away  from  the  outer  wall,  where,  in  conse- 


BURNING  CLAY  WARES. 


259 


Figure  139. 


260 


BURNING  CLAY  WARES. 


quence  of  radiation  losses  and  leakage,  it  is  essential  the  draft 
should  be  the  strongest. 

The  draft  connection  is  always  a number  of  compartments 
ahead  of  the  burning  compartment  and  one  sided  draft  has  no 
effect  on  the  burning,  but  it  does  affect  the  water-smoking 
and  heating  up  and  thus  retards  the  rate  of  progress.  To 
correct  this  one  kiln  had  the  draft  opening  in  the  outer  wall, 
then  down  and  back  under  the  kiln  to  the  central  division  wall 
and  up  into  the  main  draft  flue. 

Then  followed  the  placing  of  the  draft  flue  under  the  tun- 
nel floors  with  connections  through  the  outer  or  inner  wall; 
later  we  find  the  draft  flue  under  the  outer  wall;  and  finally 
it  is  placed  outside  the  kiln  walls  with  goose-neck  connections 
to  the  kiln. 

These  changes  are  partially  illustrated  by  Fig.  158,  Fig. 
159,  and  Fig.  160.  In  Fig.  158  we  have  the  annular  kiln,  an- 
nular smoke  flue,  and  central  stack.  In  Fig.  159  we  have  the 
early  rectangular  form  with  a longitudinal  central  draft  flue 
and  the  stack  within  the  kiln  wall.  When  the  stack  was 
placed  outside  the  kiln  wall,  the  arrangement  in  its  final  devel- 
opment for  a single  kiln  became  that  shown  in  Fig.  139,  and 
the  plan  of  the  modern  kiln  is  shown  in  Fig.  160  with  the  detail 
of  Fig.  139. 

Dampers  which  were  often  necessary  when  the  flues  were 
within  the  kiln  wall  gave  a lot  of  trouble  and  always  leaked 
more  or  less,  and  the  final  location  of  the  flues  outside  the 
kiln  walls  with  goose-neck  connections  was  the  perfect  solu- 
tion of  the  damper  problem. 

The  earlier  kiln  had  no  advanced  heating  flue  by  means 
of  which  hot  air  is  by-passed  from  the  cooling  sections  to  the 
water  smoking  sections,  thus  overcoming  or  reducing  the  scum- 
ming and  swelling  difficulties,  and  increasing  the  drying  effi- 
ciency, thereby  correspondingly  increasing  the  rate  of  burning. 
The  advanced  heating  flues  are  in  the  upper  kiln  walls  and 
connections  from  the  cooling  sections  to  the  water  smoking 
sections  are  by  means  of  hoods  covering  feed  holes  in  the  top 
of  the  tunnel  and  corresponding  holes  in  the  top  of  the  ad- 
vanced heating  flue.  This  is  illustrated  in  the  description  of 
open  top  continuous  kilns.  In  the  kiln  illustrated  the  advanced 
heating  flue  is  in  the  outer  wall,  but  it  may  be  placed  in  the 
wall  between  the  tunnels,  especially  if  the  draft  flue  is  else- 
where. Where  the  main  draft  flue  is  in  the  mid-wall,  we  may 


BURNING  CLAY  WARES. 


261 


still  have  the  advanced  heating  flue  in  the  same  wall,  either 
alongside  or  above  the  draft  flue. 

We  speak  of  sections  and  compartments  in  the  tunnel  kiln, 
but  in  the  tunnel  kiln  itself  there  are  no  divisions.  For  con- 
venience in  setting  and  unloading  the  kiln  wickets  are  spaced 
from  12  feet  to  16  feet  apart,  and  the  draft  connections  are 
correspondingly  spaced.  In  setting  the  kilns  we  fill  the  tunnel 
from  one  wicket  to  the  next,  and  blanket  the  draft  with  paper 
pasted  to  the  face  of  the  set  bricks.  This  constitutes  a section 
and  each  section  has  a draft  connection,  and  so  long  as  the 
paper  dampers  remain  in  place,  each  section  may  be  water 
smoked  by  the  hot  air  brought  from  the  cooling  sections  through 
the  advanced  heating  flue.  When  this  stage  of  the  process  is 
finished  the  connection  with  the  advanced  heating  flue  is 
broken,  and  holes  are  torn  in  the  paper  damper  nearest  the  fire 
sections  and  as  the  fire  advances,  the  damper  is  ignited  and 
thus  completely  removed. 

In  one  kiln,  perhaps  in  several  designs,  a drop  or  apron 
arch,  shown  in  the  illustration,  is  introduced  in  the  kiln  crown 
at  intervals  of  12  feet  to  16  feet.  This  arch  drops  12  inches 
to  15  inches  below  the  kiln  crown,  and  it  is  simply  a diaphragm 
to  deflect  the  gases  and  air  downward  among  the  burning  and 
cooling  bricks.  Without  this  arch,  especially  where  the  ware 
is  not  set  close  to  the  kiln  crown  and  where  the  settle  is  con- 
siderable, we  have  a continuous  free  passage  between  the  crown 
and  the  ware  along  which  the  air  will  move  and  the  purpose 
of  the  drop  arch  is  to  break  up  and  deflect  this  movement. 

Many  engineers  hold  that  all  the  heat  in  the  kiln  is  useful 
in  the  kiln  operation,  and  that  no  heat  should  be  taken  for 
outside  work.  One  prominent  tunnel  kiln  designer  provides 
openings  in  the  kiln  crown  connecting  with  wall  and  under- 
ground flues  leading  to  an  independent  dryer  and  uses  the 
heat  of  the  cooling  sections  for  drying  purposes  outside  the 
kiln  walls.  Any  heat  thus  taken  from  the  kiln,  must  be  re- 
placed by  fuel  burned  in  the  kiln,  but  in  view  of  the  fact  that 
combustion  in  the  kiln  is  absolutely  complete  and  smokeless, 
while  that  in  an  independent  furnace  is  very  imperfect,  it  is 
reasonable  that  it  is  economical  to  generate  heat  for  drying 
purposes  in  the  kiln.  Such  a kiln  will  show  higher  fuel  con- 
sumption than  a kiln  from  which  no  heat  is  taken,  but  if  we 
add  to  the  latter  the  fuel  required  to  dry  the  ware,  the  balance 
will  be  in  favor  of  the  former. 


262 


BURNING  CLAY  WARES. 


The  same  kiln  designer  introduces  furnaces  in  the  wickets 
and  these  use  cold  air,  as  do  the  furnaces  in  any  periodic  kiln. 
The  top  firing,  as  in  any  direct  coal  fired  kiln,  is  through  feed 
holes  in  the  crown,  vertical  shafts  in  the  set  ware,  into  trace 
flues  in  the  set  ware  at  the  floor  level.  These  fires  are  kept 
in  advance  and  serve  to  burn  the  ware  in  the  bottom  of  the 
kiln  and  may  be  advanced  as  rapidly  as  the  bottom  ware  can 
be  burned.  Following  these,  the  side  fires  are  started  and  the 
flame  from  them  surrounds  the  mass  of  ware,  sides  and  top 
and  completes  the  operation. 

There  is  much  to  be  said,  pro  and  con,  relative  to  such  a 
kiln  which  departs  so  radically  from  the  principles  of  an  econo- 
mizer kiln,  but  in  view  of  the  fact  that  the  kiln  has  been 
adoted  by  American  clayworkers  in  greater  degree  than  any 
other  economizer  kiln,  and  that  it  is  successfully  burning  the 
difficult  paving  brick  product,  the  merit  of  the  kiln  must  be 
conceded. 

The  self-contained  rectangular  kilns  require  that  the  ware 
shall  go  in  and  come  out  the  same  doorway  in  the  face  wall 
of  the  kiln.  If  depressed  railroad  tracks  are  put  on  each  side 
of  the  kiln  to  handle  the  burned  ware,  as  it  comes  from  the 
kiln,  and  since  transfer  cars  paralleling  the  depressed  tracks 
are  needed  to  handle  the  ware  from  the  dryer  to  the  kiln,  there 
is  considerable  interference  in  the  work. 

The  modern  kiln,  having  independent  tunnels,  with  two 
wickets  on  opposite  sides  for  each  section,  permits  the  green 
ware  to  be  handled  in  the  space  between  the  batteries,  while 
the  burned  ware  leaves  the  kiln  through  the  wickets  in  the 
outside  walls. 

The  tunnel  kiln  is  not  as  convenient  for  setting  as  the 
chambered  kiln.  The  ware  is  delivered  to  the  kiln  over  a trans- 
fer track,  and  thence  into  the  kiln  on  a spur  track,  and  from 
this  to  a transfer  car  inside  the  kiln  to  the  working  face. 
When  a car  is  unloaded  it  must  be  transferred  back  to  the 
wicket,  and  out  before  another  load  can  be  brought  in.  In  some 
instances  a portable  turntable  is  used  in  the  kiln  in  the  place 
of  the  kiln  transfer  track  and  car,  but  this  is  simply  a matter 
of  preference  and  is  without  effect  on  the  periodic  setting 
operations. 

The  compartment  kiln  with  wide  wickets  permits  the  use 
of  a double  track  as  in  rectangular  periodic  kilns,  and  no  time 
need  be  lost  by  the  setting  gang  in  consequence  of  switching 
cars  in  and  out. 


BURNING  CLAY  WARES. 


263 


The  setting  in  tunnel  kilns  requires  trace  flues  in  the  bottom 
and  vertical  flues  corresponding  with  the  top  feed  holes.  A 
similar  flue  system  in  the  set  ware  is  required  in  the  direct 
coal  fired  chambered  type  of  kilns,  but  the  gas  fired  chambered 
kiln  has  the  same  setting  as  in  a down-draft  kiln. 

A tunnel  kiln  should  not  have  fewer  than  16  sections  and 
preferably  20  or  22  sections.  From  6 to  9 sections  are  cooling, 
2 to  3 sections  burning,  3 to  4 sections  heating  up,  2 to  3 sec- 
tions water-smoking  and  two  or  more  sections  for  working 
space.  Occasionally  a kiln  is  built  of  such  a number  of  sec- 
tions that  it  may  be  double  fired,  or  three,  four  or  five  sets 
of  fires,  but  such  kilns  are  merely  combinations  of  several  kilns 
in  a single  construction,  and  each  fire  has  its  related  cooling, 
heating  up,  water-smoking,  and  working  sections,  and  it  is  oper- 
ated as  an  independent  kiln. 

A description  of  the  many  ring  kilns  tried  out,  in  use,  or 
on  the  market  in  this  country  would  be  voluminous.  They 
all  start  with  the  Hoffman  kiln  and  the  improvements  consist 
of  a rearrangement  of  the  air  and  gas  flues,  the  addition  of 
advanced  heating  flues,  drop  arches,  perforated  floors,  perma- 
nent trace  flues,  outside  furnaces,  adaptation  to  producer  gas, 
oil,  etc.  We  do  not  wish  to  intimate  that  the  changes  and 
additions  have  not  improved  the  kiln — quite  the  contrary.  The 
kiln  in  its  highest  development  occupies  a prominent  position 
in  the  clayworking  industries  at  the  present  time,  probably 
ranking  first  among  economizer  kilns  in  the  quantity  of  output. 

Zig-Zag  Kiln. 

The  zig-zag  kiln  is  merely  a rearrangement  of  the  ring  kiln, 
giving  greater  compactness.  Such  a kiln  is  illustrated  in  Fig. 
140.  There  are  several  modifications  of  this  type  of  kiln  both 
in  the  arrangement  of  the  several  pseudo  chambers,  and  in  the 
floor  and  draft  exits.  In  some  the  plan  is  simply  a single 
battery,  with  an  underground  return  flue. 

This  has  the  advantage  that  the  ware  enters  on  one  side 
and  is  removed  from  the  opposite  side. 

In  other  plans  the  battery  of  compartments  is  single,  but 
the  return  flue  is  above  ground  and  has  the  same  cross  section 
as  the  other  compartments,  in  other  words,  a longitudinal  com- 
partment connects  the  ends  of  the  battery  of  compartments, 
making  the  operation  of  the  kiln  fully  continuous. 

Others  are  the  double  battery  type,  shown  in  the  illustra- 
tion. 


264 


BURNING  CLAY  WARES. 


The  kiln  never  found  favor  in  this  country,  so  far  as  we 
know,  and  a detailed  description  of  any  of  the  type  would  not 
interest  American  clayworkers.  It  seems  strange  that  the 
zig-zag  kiln,  with  its  greater  compactness  and  lower  radiation 
loss,  has  not  found  some  acceptance  in  this  country. 

Compartment  Kilns. 

The  compartment  kiln  is  in  a measure  a development  of 
the  tunnel  kiln.  The  earlier  types  of  tunnel  kilns  were  direct 
coal  fired  and  the  burning  fuel  came  in  contact  with  the  ware. 
This  would  not  be  objectionable  for  a number  of  common  wares, 


L9_  _i . _9 1 1 _ ~ i~_  p_  i"_v. 


Fig.  140. 

but  for  other  wares  it  was  essential  that  the  fuel  be  burned 
in  a separate  compartment. 

The  simplest  and  earliest  effort  to  separate  the  high-grade 
ware  from  the  fuel  was  to  build  the  trace  and  vertical  firing 
flues  of  common  bricks  and  to  fill  the  space  between  with  the 
higher  grade  ware,  but  this  involved  the  manufacture  of  two 
or  more  kinds  of  ware  simultaneously,  complicated  the  setting 
and  increased  the  labor  cost. 

The  first  step  toward  a compartment  kiln  was  to  introduce, 
in  the  ring  kiln,  division  walls  at  intervals,  the  walls  being 
double,  with  space  between.  In  this  space  were  placed  so- 
called  step  grates  upon  which  the  fuel  collected  and  openings 
in  the  division  walls  provided  for  the  admission  of  air  on  one 
side  and  the  exit  of  the  gases  on  the  other. 


BURNING  CLAY  WARES. 


265 


Fig.  141,  view  plan  below  the  crown,  and  Fig.  148,  elevation, 
illustrate  such  a division  wall-firing  pocket. 

In  a modern  American  ring  kiln  drop  or  apron  arches  are 
introduced  in  the  tunnel  crown  at  suitable  compartment  inter- 
vals. The  green  bricks  are  set  in  front  of  the  aprons  to  form 


Fig.  141. 


Fig.  142. 


a solid  wall  except  openings  through  the  base  corresponding  to 
trace  flues.  This  green  brick  wall  is  virtually  a division  wall. 
The  bricks  may  shrink  and  the  wall  settle  to  the  depth  of  the 
deep  arch  without  opening  a direct  passage  from  compartment 


266 


BURNING  CLAY  WARES. 


to  compartment.  Back  of  each  apron  wall  are  slots  in  the 
kiln  side  walls,  and  in  line  with  these  and  inserted  into  them 
is  a second  solid  green  brick  wall  across  the  tunnel,  but  drop 
ping  off  below  the  kiln  crown.  This  second  wall  is  built  of 
bricks  set  herring  bone  fashion  so  that  under  fire  shrinkage 
the  bricks  will  settle  together  and  maintain  a relatively  tight 
wall.  The  extension  of  this  wall  into  the  side  wall  slots  pre- 
vents any  openings  around  the  ends  of  the  walls.  This  second 
wall  diverts  the  gases  from  the  trace  flues  to  the  kiln  crown, 
and  the  initial  wall  brings  them  back  to  the  trace  flues  in  the 
next  compartment.  The  firing  is  with  producer  gas  introduced 
through  a series  of  ports  a short  distance  back  of  the  deflecting 
division  wall. 

Fig.  143,  longitudinal  section,  illustrates  the  above  described 
division  of  a ring  kiln  into  compartments. 

An  English  kiln  introduced  into  this  country  divided  the 


Fig.  143. 


tunnel  into  a series  of  compartments  by  permanent  division 
walls.  Each  division  wall  was  perforated  at  the  base  by  a 
number  of  ports,  thus  connecting  the  compartments. 

In  front  of  each  division  wall ; that  is,  on  the  firing  side, 
was  placed  a low  box  to  receive  the  fuel.  A section  through 
one  port  in  the  base  of  the  division  wall  is  shown  in  Fig.  144. 

The  hot  air  from  the  rear  cooling  compartment  is  split  as 
it  passes  through  the  division  wall  ports.  Part  of  it  enters  the 
fuel  box  at  the  floor  level  and  is  drawn  up  through  the  fuel, 
thus  giving  the  primary  combustion.  The  second  portion  enters 
above  the  fuel  box  and  combines  with  the  products  of  primary 
combustion.  The  operation  is  that  of  a gas  producer,  and  it 
may  be  said  that,  crude  and  simple  as  it  was,  little  fault  could 


BURNING  CLAY  WARES. 


267 


Fig.  145. 


268 


BURNING  CLAY  WARES. 


be  found  with  this  feature  of  the  kiln.  In  a subsequent  con- 
struction, the  division  walls  became  the  support  of  the  com- 
partment arches,  following  the  usual  construction  of  a com- 
partment kiln. 

Another  (German)  method  of  combustion  apart  from  the 
ware  is  shown  in  Fig.  145,  a view  above  the  floor  on  the  left, 


Fig.  146. 


and  similarly  below  the  floor  on  the  right,  and  Fig.  146,  a 
vertical  section.  Here  we  have  side  firing  spaces  with  firing 
plates  to  receive  the  fuel,  similar  to  the  step  grates,  with  the 
hot  air  entering  from  below.  The  use  of  step  grates  is  com- 
mon practice  in  Germany,  if  we  may  judge  from  the  literature, 
while  in  this  country  they  have  not  proven  successful. 

A German  counterpart  of  the  English  kiln  (Fig.  144)  is 
illustrated  in  Fig.  147  and  Fig.  148.  It  was  necessary  to  use 
lump  coal  in  the  English  kiln,  whereas  in  the  German  kiln  with 


BURNING  CLAY  WARES. 


269 


the  step  grates,  powdered,  or  at  least  fine  coal  is  essential, 
and  a further  essential  is  that  the  coal  should  be  easily  in- 
flammable and  highly  gaseous.  Brown  coal,  lignite  and  similar 
volatile  and  gaseous  fuels  are  especially  adapted  to  the  step 
grate  combustion  furnaces.  This  is  probably  one  reason  why 
the  step  grates  in  any  application  have  not  found  favor  in  this 
country. 

In  the  German  kiln  above  illustrated  the  coal  is  dropped 
through  feed  holes  in  the  kiln  crown  and  collects  on  the  fire 
clay  plates.  Hot  air  from  cooling  compartments  in  the  rear 
enters  the  burning  compartment  through  the  ports  in  the  divi- 
sion wall  above  and  below  the  plates  holding  the  fuel.  The  hot 
air  from  below  heats  the  plates,  gasifies  the  coal  and  finally 
unites  with  the  residual  carbon  in  its  passage  over  the  top  of 
the  plates. 

The  secondary  hot  air  from  the  upper  ports  completes  the 


Fig.  149. 


combustion.  The  operation  is  crude,  but  the  final  results  are 
perfect  in  so  far  as  complete  combustion  is  concerned. 

The  step  grate  principle  in  other  kilns  is  further  carried 
out  by  increasing  the  height  of  the  fire  wall  and  putting  in 
a series  of  step  grates,  as  shown  in  Fig.  142  and  Fig.  147. 

The  next  advance  is  the  double  fired  step  grate  kiln,  shown 
in  Fig.  149.  The  arrows  show  the  movement  of  the  air  under 
the  floor,  to  the  fire  bags,  thence  up  around  the  step  grates, 
over  into  the  kiln,  and  down  through  the  ware  and  kiln  floor, 
repeating  the  operation  in  the  succeeding  compartments  until 
the  draft  outlet  is  reached.  Any  clayworker  will  understand 
that  the  distributing  air  flues  in  any  compartment,  one  of  which 
is  shown  in  the  illustration,  alternate  with  perforated  floor 


270 


BURNING  CLAY  WARES. 


flues  leading  into  the  under  main  draft  flue,  thence  forward 
into  the  distributing  flues  in  the  next  compartment.  The  col- 
lecting flue  in  one  compartment  becomes  the  distributing  flue 
in  the  next  compartment. 

The  same  principle  of  collection  and  distribution  of  the  air 
and  combustion  gases  is  used  in  later  producer  gas  double  fired 
kilns  with  marked  success. 

An  early  kiln  developed  in  this  country  is  shown  in  Fig. 


150,  section,  and  Fig.  151,  a partial  plan  of  two  compartments 
of  the  kiln  including  the  central  longitudinal  division  wall. 
The  floor  is  solid.  The  division  walls  have  ports  in  the  bottom 
spaced  about  27  inches.  The  draft  connection  is  in  the  corner 
of  each  compartment,  with  an  up-take  flue  into  the  bottom  of 
a main  draft  flue.  The  kiln  is  simplicity  itself.  The  ware  is 
set  away  from  the  division  walls  on  each  side,  thus  forming 
combustion  spaces  in  which  the  firing  is  partly  done  through 
the  outer  feed  holes.  Under  the  intermediate  holes  the  ware 


BURNING  CLAY  WARES. 


271 


is  set  to  form  step  grates,  thus  spreading  out  the  fuel  fan- 
wise  in  the  mass  of  ware  from  top  to  bottom. 

The  combustion  gases  from  the  lateral  spaces  are  partly 
drawn  through  the  lower  courses  of  the  ware  by  means  of 
trace  flues  set  in  the  ware,  and  partly  pass  over  the  top  of 
the  ware,  thus  burning  the  top  and  bottom.  The  fuel  within 
the  mass  of  ware  performs  its  wTork  locally,  and  a fairly  uni- 
form result  is  obtained  throughout.  These  results,  however, 
depend  largely  upon  intelligent  setting  and  firing. 

A weakness  of  compartment  kilns  is  the  tendency  of  the 
division  walls  to  lean  in  the  direction  of  the  draft,  and  in 
consequence  throw  the  arch,  as  shown  in  Fig.  152,  and  after 
a few  years’  use  the  kiln  has  to  be  rebuilt,  which,  considering 
the  cost,  is  a serious  matter.  The  life  of  the  kiln  depends 
upon  the  thickness  and  solidity  of  the  division  walls,  their 


height,  the  quality  of  the  structural  material  and  workmanship, 
the  rise  of  the  crown  and  the  temperatures  attained. 

It  has  been  suggested  that  the  kiln  should  be  designed  so 
that  the  firing  can  be  reversed  annually  or  semi-annually. 

If  a kiln  is  well  built  the  slight  displacement  which  would 
occur  in  six  months  would  be  corrected  by  a reversal  of  the 
direction  of  the  firing  for  a like  period  or  longer,  as  may  be 
required.  A simple  construction,  such  as  shown  in  Fig.  150, 
could  easily  be  designed  for  such  a reversal  of  the  draft.  It 
would  only  be  necessary  to  carry  the  compartment  draft  con- 
nections through  the  middle  wall,  as  shown  by  the  dotted  lines 
in  Fig.  151,  and  block  them  up  on  one  side  or  the  other  with 
temporary  walls.  A reversal  of  the  firing  then  would  only 
require  that  the  kiln  be  burned  off  and  emptied,  immediately 
refilling  and  firing  in  the  opposite  direction,  changing,  of  course, 
the  temporary  wralls  blocking  the  draft  ports. 

In  a kiln  with  the  draft  connections  in  the  corners  of  the 


272 


BURNING  CLAY  WARES. 


compartments,  one  would  say  that  there  would  be  a diagonal 
tendency  in  the  movement  of  the  kiln  gases.  This  is  true, 
but  it  does  not  materially  affect  the  burning  compartment, 
because  the  draft  control  is  always  several  compartments  ahead 
of  the  firing  compartment.  The  cooling  effect  of  the  wicket 
and  outside  wall  forming  the  end  of  the  compartment  would 
result  in  underburned  ware  were  it  not  for  heavier  firing  in 
the  end  feed  holes,  and  even  then  we  often  do  not  get  the  ends 
as  fully  burned  as  the  remainder  of  the  compartment. 

The  diagonal  draft  does  effect  the  water-smoking  and  heat- 


ing up  and  lagging  in  this  work  often  continues  through  the 
burning. 

Fig.  153,  section,  and  Fig.  154,  view  plan  of  the  division 
wall  between  compartments,  illustrate  a modification  of  the 
preceding  kiln  to  eliminate  the  diagonal  draft.  Between  the 
ports  in  the  division  wall  connecting  the  compartments  are 
up-take  flues  into  the  bottom  of  a cross  collecting  flue  which 
in  turn  enters  a longitudinal  main  draft  flue.  If  the  sizes  of 
the  division  wall  flues  are  properly  proportioned  we  are 
assured  of  uniform  draft  from  end  to  end  of  the  burning  com- 
partment. It  would,  of  course,  be  a simple  matter  to  provide 
dampers  for  each  of  the  up-take  draft  flues,  but  this  is  not  con- 
sidered necessary.  Even  imperfect  operation  so  far  breaks  up 


BURNING  CLAY  WARES. 


273 


the  diagonal  tendency  that  any  material  or  noticeable  retarda- 
tion is  impossible. 

Another  method  of  overcoming  the  diagonal  draft  and  at 
the  same  time  in  some  degree  counteract  the  cooling  effect  of 
the  outside  wall,  is  to  put  a draft  outlet  in  the  outer  as  well 
as  in  the  inner  wall  with  a down-take  from  the  former  to  an 
under-ground  cross  flue  leading  to  and  connecting  with  the 
main  draft  flue,  or  an  up-take  to  an  upper  cross  flue.  Such 


a flue  arrangement  with  proper  draft  control  enables  us  to 
have  the  draft  all  to  the  outer  wall,  all  to  the  inner  wall,  or 
balanced  as  may  be  desired.  When  we  consider  that  any 
damper  within  a kiln  structure  leaks  more  or  less,  and  in  con- 
sequence there  is  always  some  draft  through  the  damper  con- 


Figure  155. 


nection  of  the  burning  compartment,  it  becomes  more  important 
that  the  tendency  of  the  draft  should  be  toward  the  outer  wall, 
where  radiation  losses  are  the  greatest. 

In  kilns  which  have  been  designed  and  put  in  use,  one  can 
generally  trace  a development  from  the  simpler  form  to  over- 
come some  of  the  difficulties,  but  occasionally  there  is  a marked 
departure  from  earlier  types. 

Fig.  155,  section  of  the  kiln,  Fig.  156,  plan,  and  Fig.  157, 


274 


BURNING  CLAY  WARES. 


section  of  the  division  wall,  show  an  American  kiln  which 
differs  materially  from  the  general  plan. 

The  movement  of  the  air  and  gases  in  the  compartments 
is  longitudinally  of  the  compartment,  reversing  in  the  succeed- 


Figure  156. 


ing  compartment,  in  other  words,  it  is  that  of  a zig-zag  kiln, 
and  it  could  very  properly  be  termed  a zig-zag  compartment 
kiln. 

Presumably  the  firing  in  each  compartment  is  more  or  less 
progressive.  As  the  fires  slacken,  or  cease  in  the  entering 


end  of  the  firing  compartment,  air  becomes  available  to  start 
or  increase  the  fires  toward  the  other  end. 

The  connection  from  compartment  to  compartment  is  a 
single  opening  at  one  end  of  each  division  wall,  alternating  so 


BURNING  CLAY  WARES. 


275 


that  the  movement  of  the  air  and  gases  is  forward  through 
one  compartment  and  backward  through  the  next,  continuing 
thus  from  the  entering  cooling  compartment  to  the  exit  water- 
smoking compartment.  The  collecting  draft  flues  are  between 
kiln  crown  arches,  above  the  crowns,  immediately  over  the 
division  walls,  but  each  extends  only  half  the  length  of  the 
compartments,  and  they  alternate  in  the  successive  compart- 
ments. These  flues  are  connected  on  the  underside  of  the  kiln 
crown  to  the  adjacent  firing  holes,  and  at  the  outer  ends  con- 
nect with  a down-take  flue  in  the  outside  wall,  thence  into  an 
under-ground  main  draft  flue. 

The  compartment  connection  is  in  one  end  of  the  division 
wall  and  the  draft  connection  is  in  the  other  end,  the  position 
alternating  in  the  consecutive  walls. 

Trace  and  vertical  firing  flues  are  set  in  the  ware  and  the 
firing  is  the  usual  top  coal  operation,  with  the  addition  that, 
through  holes  in  the  side  and  wicket  walls  corresponding  with 
the  trace  flues  in  the  ware,  the  accumulations  of  ashes  in  the 
trace  flues  under  the  feed  hole  flues  can  be  raked  out,  or  lev- 
eled down  and  thus  keep  the  active  combustion  near  the  kiln 
floor  and  provide  free  passage  for  hot  air  through  the  lower 
courses  of  the  ware  at  all  times. 

The  operation  of  the  kiln  is  in  outline  as  follows : 

The  air  enters  as  usual  through  the  compartment  from 
which  the  ware  is  being  drawn,  and  traverses  the  several 
cooling  compartments  longitudinally,  forward  and  backward, 
finally  entering  one  end  of  the  burning  compartment  where  it 
comes  in  contact  with  the  fuel.  The  combustion  gases  continue 
the  forward  movement  through  the  heating-up  compartment 
into  the  water-smoking  compartment,  where  they  are  drawn 
off  through  the  connected  feed  hole  shafts  into  the  collecting 
draft  flue,  thence  to  the  down-take  and  main  draft  flue.  The 
top  draft  is  claimed  as  a feature  of  the  kiln,  in  that  as  the 
vapor  rises  from  the  water-smoking  ware  it  is  drawn  off  to 
the  stack  instead  of  being  drawn  down  through  the  ware  and 
condensing  thereon  as  frequently  happens  in  down-draft  opera- 
tion. 


Kiln  Arrangement. 

A factory  should  be  designed  so  that  there  will  be  a con- 
tinuous forward  movement  from  the  clay  supply  to  the  car 
loaded  with  the  finished  ware.  This  does  not  mean  that  the 
operation  must  be  strung  out  in  a line,  quite  the  contrary,  but 


276 


BURNING  CLAY  WARES. 


it  does  mean  that  the  product  shall  not  cross  itself  at  any 
point  and  thus  interfere  with  the  general  forward  movement. 

The  original  ring  kiln  was  annular  (Fig.  158)  and  decidedly 
awkward  for  the  movement  of  ware  which  can  be  most  eco- 
nomically moved  in  straight  lines  and  right  turns. 

These  early  kilns  were  probably  filled  and  emptied  by  means 
of  wheelbarrows,  or  perhaps  the  ware  was  carried  in  and  out 
by  hand.  A circular  track  could  be  put  around  the  kiln  with  a 
turntable  opposite  each  doorway,  or  better,  a circular  transfer 
track  with  a turntable  car,  but  with  any  arrangement  the  kiln 
is  not  adapted  to  modern  methods. 

The  plan  was  changed  to  an  oblong  shape  (Fig.  159), 
which  is  an  improvement,  but  still  there  is  more  or  less  inter- 
ference in  the  setting  and  unloading  since  the  ware  must  come 


Fig.  158. 


Fig.  159. 


out  the  same  doorway  through  which  it  is  taken  in  and  the 
setting  and  unloading  operations  are  not  far  apart.  This  double 
operation  on  the  same  side  of  the  kiln  is  complicated  by  the 
fact  that  the  transfer  tracks  for  green  ware  are  slightly  de- 
pressed and  the  loading  tracks  for  the  finished  ware  deeply 
depressed.  One  or  the  other  must  be  crossed  by  the  moving 
ware,  and  there  are  frequent  delays  and  numerous  minor  acci- 
dents. 

The  final  and  satisfactory  plan  (Fig.  160)  has  the  tunnels 
separated  and  the  ware  enters  through  the  inner  doorways 
and  is  taken  out  through  the  outer  doorways. 

The  early  compartment  kilns  (Fig.  161)  have  the  same  diffi- 
culty as  the  ring  kilns  shown  in  Fig.  159,  and  to  overcome  this, 
particularly  where  it  was  impractical  to  have  a spur  track  on 
each  side  of  the  kiln,  so-called  semi-continuous  kilns  were  built 
(Fig.  162)  and  the  operation  became  continuous  by  means 


BURNING  CLAY  WARES. 


277 


of  a return  flue  connecting  the  ends  of  the  kiln.  It  is  prac- 
tical to  build  a kiln  with  ten  or  even  twelve  compartments  and 
get  satisfactory  operation  through  such  a return  flue. 

It  is  but  an  extension  of  the  semi-continuous  kiln  to  build 


Fig.  161. 


278 


BURNING  CLAY  WARES. 


a parallel  duplicate  battery  of  compartments,  connecting  the  two 
batteries  of  compartments  by  cross-over  flues  and  thus  get  the 
fully  continuous  kiln  as  shown  in  the  dotted  lines  in  Fig.  162, 
which  gives  the  same  yard  arrangement  as  Fig.  160. 

The  zig-zag  kiln  illustrated  in  Fig.  140  has  a large  part 
of  the  ware  coming  out  the  same  doorway  through  which  it 
entered,  but  a single  battery  kiln  with  a return  flue  may  be 
built  as  shown  in  Fig.  163.  Such  a kiln  develops  a very  long 


T 

V 


Fig.  162. 

tunnel  in  a comparatively  short  space — in  other  words,  a com- 
pact kiln  and  factory  arrangement. 

Producer  Gas  Economizer  Kilns. 

We  do  not  know  when  producer  gas  was  first  used  in  this 
country  in  the  production  of  clay  wares.  At  Mt.  Savage,  Md., 
there  was  in  operation  in  the  early  eighties,  and  up  to  within 
a few  years  ago,  a tunnel  kiln  fired  with  producer  gas. 

The  producer  was  built  on  a car  and  traveled  forward  as 
the  fires  progressed.  It  was  scarcely  more  than  a large  port- 
able box  furnace,  without,  it  is  our  recollection  either  steam 
or  air  pressure,  in  which  the  combustion  was  very  imperfect 


BURNING  CLAY  WARES. 


279 


from  either  the  standpoint  of  complete  combustion  or  producer 
gas  development,  but  being  movable  and  at  all  times  in  close 
touch  with  the  burning  compartment,  thus  getting  full  benefit 
of  the  sensible  heat  in  the  combustion  gases,  it  was  not  impor- 
tant that  a high-grade  gas  be  developed. 

Producer  gas  operation  in  foreign  countries  was  much 
earlier  than  in  this  country.  Mendheim  developed  a producer 
gas  kiln  and  put  it  in  operation  in  1867.  The  first  kiln  was 
very  simple.  If  we  were  to  take  the  kiln  illustrated  in  Fig. 
150  and  introduce  a gas  flue  under  or  in  front  of  the  division 
wall  below  the  kiln  floor  level,  with  ports  in  the  crown  of  this 
flue  to  mate  with  the  hot  air  ports  in  the  division  wall,  and 
build  a bag  wall  in  front  of  the  division  wall,  we  will  have  the 
first  type  of  producer  gas  compartment  kiln. 

The  early  introduction  of  producer  gas  in  tunnel  kilns  was 


1 


Fig.  163. 

equally  simple,  and  in  this  operation  it  is  doubtful  whether  we 
have  progressed  any. 

In  the  early  installation  a series  of  cross  fines  carry  the 
gas  from  the  main  gas  flue  outside  the  kiln  wall  to  and  under 
the  tunnel  floor  and  ports  in  the  top  of  these  flues  correspond- 
ing to  the  ordinary  feed  holes  in  the  kiln  crown  deliver  the  gas 
into  the  kiln  tunnel.  As  the  setting  of  the  ware  progresses 
each  gas  port  is  converted  into  a vertical  flue  to  the  top  of 
the  setting  by  the  use  of  perforated  clay  pipes  closed  at  the 
top.  The  combustion  of  the  gas  jets  from  the  pipes  is  com- 
pleted by  the  hot  air  coming  forward  through  the  cooling  ware 
in  the  tunnel. 

This  is  a German  method  described  by  Schmatolla. 


280 


BURNING  CLAY  WARES. 


Bock  shows  the  same  method  except  that  the  gas  is  intro- 
duced from  above  through  the  feed  holes. 

An  American  method  shown  in  Fig.  143  and  described  in 
connection  therewith  introduces  the  gas  through  a series  of 
ports  in  the  kiln  crown  from  side  wall  to  side  wall.  A drop 
brick  or  baffle  in  front  of  each  port  serves  to  spread  the  gas 
and  air.  The  heat  is  forced  to  the  bottom  of  the  kiln,  as  shown 
and  described. 

The  original  Mendheim  compartment  kiln,  which  has  been 
repeatedly  illustrated  and  described,  carries  the  air  forward 
under  the  division  wall  and  across  the  compartment  in  a series 
of  parallel  flues.  Between  these  are  gas  flues  supplied  with  gas 
from  under  cross  flues  connecting  with  outside  main  gas  flue. 
The  gas  and  air  are  brought  together  through  small  openings 
in  the  kiln  floor.  The  firing  is  over  all  parts  of  the  kiln  floor 
and  in  direct  contact  with  the  ware,  resulting  in  overbumed 


ware  in  the  bottom  and  underburned  ware  on  top.  This  type 
of  kiln  has  been  superseded  by  the  later  and  more  satisfactory 
down-draft  types  of  kilns.  Such  a later  kiln  is  illustrated  in 
Fig.  164,  section,  and  Fig.  165,  view  plan.  The  gas  is  delivered 
around  the  kiln  in  underground  flues,  and  cross  flues  under  the 
kiln  floor  deliver  the  gas  to  ports  in  the  bottom  of  the  bags. 
The  hot  air  is  brought  forward  from  the  cooling  compartments 
through  underfloor  flues  corresponding  with  the  gas  ports.  The 
draft  connection  is  at  the  floor  level  in  the  corner  of  each  com- 
partment. 

The  Dunnachie,  a Scotch  kiln,  Fig.  166,  had  three  installa- 
tions in  this  country  in  1891 ; one,  of  two  kilns,  in  Pittsburgh, 
Pa.,  and  one  in  Portsmouth,  O.,  for  fire  bricks,  and  one  in  Perth 
Amboy,  N.  J.,  for  terra  cotta,  but  all  of  them  have  long  since 
been  dismantled.  The  gas  was  introduced  through  a single 


BURNING  CLAY  WARES. 


281 


flue  across  each  compartment,  instead  of  two  flues  as  in  the 
previously  described  kiln.  There  was  no  combustion  bag,  and 
instead  of  four  gas  ports  there  were  a series  of  small  ports  in 
the  kiln  floor  immediately  in  front  of  the  division  wall.  The 
air  for  combustion  was  collected  behind  the  division  wall,  as 
in  Fig.  164,  with  the  difference  that  the  transverse  flue  was 
under  the  division  wall  and  had  two  damper  controlled  ports 
through  which  the  air  was  delivered  into  an  upper  division 
wall  transverse  flue  from  which  at  the  floor  level  were  a series 
of  ports  corresponding  with  the  gas  ports.  The  air  entered 


horizontally,  and  projecting  into  each  stream  of  air  there  was 
a vertical  jet  of  gas  from  below.  The  ware  was  set  away  from 
the  division  wall  to  form  a combustion  space  equivalent  to  a 
bag  space.  The  illustration  and  description  applies  to  the  kiln 
as  introduced  into  this  country  in  1891,  and  we  do  not  know 
what  changes  and  improvements  may  have  been  made  in  the 
kiln  up  to  date,  but  so  far  as  we  know  the  three  kilns  men- 
tioned were  the  only  ones  ever  tried  out  in  this  country. 

The  Youngren  (English)  kilns,  illustrated  in  Fig.  167,  longi- 


282 


BURNING  CLAY  WARES. 


tudinal  section,  and  Fig.  168,  cross  section,  introduced  into  this 
country  a number  of  years  ago,  marks  the  beginning  of  suc- 
cessful operation  of  producer  gas  fired  compartment  econo- 
mizer kilns.  Whatever  the  merit  of  a foreign  kiln  it  requires 
time,  patience,  ability  and  money  to  adapt  it  to  Ainerican  con- 
ditions. We  took  English  and  German  plans  worked  out  for 
capacities  of  30,000  to  60,000  bricks  per  week  and  simply  en- 


Fig.  167. 


larged  them  to  get  similar  outputs  per  day,  and  the  result 
in  most  instances  were  disastrous. 

Mr.  Youngren’s  unbounded  faith  in  his  kiln,  his  recognition 
and  admission  of  its  faults,  his  ability  to  overcome  such  faults, 
and  his  untiring  energy  in  presenting  the  kiln  to  American 
clayworkers  have  won  for  the  kiln  the  prominent  position  it 
holds  today. 

The  number  of  kilns  of  the  Youngren  type  which  have  been 


BURNING  CLAY  WARES. 


283 


built  in  this  country  has  given  opportunity  to  develop  the 
faults  and  correct  them  in  so  far  as  the  type  will  permit  and 
to  bring  the  kiln  to  its  highest  efficiency. 

The  kiln  has  been  improved  not  only  by  those  promoting  it, 
but  by  those  operating  it,  and  at  the  present  time  there  are 
more  installations  of  Youngren  kilns  under  several  names  than 
any  other  compartment  continuous  kiln.  A number  of  the 
designers  of  compartment  kilns  on  the  market  have  been  oper- 
ators of  Youngren  kilns  and  their  kilns  are  extensions  of  and 
improvements  on  the  Youngren  kiln.  The  promoters  of  the 
Youngren  kiln  have  developed  new  plans  which  are  patented, 
yet  the  ground  work  of  these  new  kilns  must  be  credited  to 
Youngren,  although  they  must  be  counted  as  improvements  and 
not  as  infringements.  The  most  marked  advance  is  in  the 
double  fired  type. 

The  Youngren  kiln  is  illustrated  in  Fig.  167,  section,  and 


Fig.  168,  section  of  the  division  wall.  The  gas  is  carried 
through  a pipe  on  the  kiln  wall  or  a flue  within  the  kiln  wall, 
and  delivered  to  cross  flues  between  the  kiln  arches  below  the 
kiln  pavement  through  a goose-neck  conection  or  by  means  of 
a valve.  The  position  of  the  gas  flue  will  depend  upon  whether 
the  kiln  is  a single  battery  of  compartments  (semi-continuous), 
a double  battery  in  a single,  unit,  or  a double  battery  in  two 
units.  In  the  modern  gas  fired  kiln  we  must  have  (1)  a main 
draft  flue  the  full  length  of  the  kiln  and  extending  to  the  draft 
fan,  (2)  cross  draft  flues  for  each  compartment,  (3)  a main 
gas  flue,  (4)  cross  gas  flues  to  each  compartment,  (5)  an  ad- 
vanced heating  flue,  (6)  cross-over  flues  at  each  end  connect- 
ing the  batteries,  or  a return  flue  connecting  the  ends  of  a 
single  battery.  The  double  unit  type  is  particularly  compli- 
cated, in  that  there  must  be  two  cross-over  flues  at  each  end 


284 


BURNING  CLAY  WARES. 


— one  connecting  the  compartments  and  one  continuing  the 
advanced  heating  fine.  Such  a multiplicity  of  flues  requires 
varied  arrangements  to  cover  the  several  modifications  of  the 
modern  kiln. 

In  the  Youngren  kiln  the  flues  delivering  the  gas  from  the 
main  gas  flue  to  the  several  compartments  are  centered  in 
the  spandrels  of  the  crown  arches.  From  the  bottom  of  these 
flues  are  down-take  flues  in  the  division  walls,  with  ports  into 
the  bags  at  the  base.  Each  of  these  individual  gas  flues  is  con- 
trolled by  a valve.  The  original  Youngren  kiln,  in  order  to 
reduce  the  number  of  control  valves,  branched  the  individual 
gas  flues  so  that  each  supplied  four  ports,  or,  in  other  words, 
four  fires,  as  shown  on  the  left  of  the  sectional  drawing,  Fig. 
168.  The  modern  kiln  uses  individual  straight  flues  for  each 
fire,  as  shown  in  the  illustration,  and  the  reason  for  this  is 
evident  to  any  one  who  has  had  experience  in  the  use  of  pro- 
ducer gas.  It  is  doubtful  whether  we  should  consider  the  mod- 
ern kiln  as  a Youngren  kiln.  The  kiln  has  been  modified  and 
improved  and  such  improvements  covered  by  patents,  some  of 
which  are  recent  (1918). 

The  air  from  the  cooling  compartments,  in  the  original  plan, 
is  drawn  down  through  the  ware,  though  a perforated  floor, 
into  parallel  collecting  flues  leading  forward  through  the  divi- 
sion wall  into  the  base  of  the  bag,  thus  delivering  the  hot  air 
below  the  gas  port. 

A question  discussed  by  engineers  is  whether  the  gas  should 
be  below  the  air,  or  vice  versa.  It  is  generally  conceded  that 
the  lighter  fluid  should  be  underneath.  The  maximum  flow  of 
a gas  in  a vertical  flue  is  in  the  center.  If  the  lighter  gas  enters 
the  flue  above  the  heavier  it  will  pre-empt  the  center  of  the  flue 
and  the  heavier  gas  will  be  drawn  up  as  an  envelope  of  the 
lighter  gas.  On  the  other  hand,  if  the  lighter  gas  is  below  the 
heavier  it  will,  because  of  its  greater  force,  overtake  the 
heavier  gas  and  crowd  it  aside  to  get  into  the  path  of  least 
resistance,  which  is  the  center  of  the  flue.  In  this  way  there 
is  a more  intimate  mixture  of  gas  and  air. 

It  is  generally  assumed  that  gas  is  lighter  than  air  and  con- 
sequently should  be  under  the  air  in  a combustion  operation. 
At  62  deg.  F.  air  weighs  .075  pound  per  cubic  foot  and  producer 
gas  about  .065  pound.  If  the  air  enters  at  1,500  deg.  F.  and 
the  gast  at  1,000  deg.,  the  relative  weights  are  .0196  and  .0216 
— the  air  being  the  lighter,  and  theoretically  should  be  under 
the  gas.  It  is  the  author’s  opinion  that  it  makes  little  or  no 


BURNING  CLAY  WARES. 


285 


difference,  and  one  might  present  arguments  favorable  to  put- 
ting the  heavier  gas  under  the  lighter. 

For  example,  complete  combustion  at  the  entry  ports  is  not 
desirable,  because  it  generates  an  intense  heat  at  those  points, 
and  it  is  difficult  to  get  the  heat  over  into  the  kiln.  In  many 
instances  it  is  desirable  to  carry  the  heat  in  part  latently  from 
the  furnace  to  the  ware  and  complete  the  development  in  con- 
tact with  the  ware. 

The  modern  kiln  developed  from  the  Youngren,  but  no  longer 
the  Youngren,  is  a marked  advance  over  the  kiln,  as  shown  in 
Fig.  167.  In  that  illustration  it  is  seen  that  the  hot  air  or  gas 
passes  from  one  compartment  into  the  next,  and  if  it  were 
desired  it  would  be  practical  to  have  fully  perforated  floors. 

In  one  recent  plan  the  under-floor  collecting  flues  are  so 
arranged  that  air  from  cooling  compartments  may  be  deliv- 
ered to  at  least  two  burning  compartments,  and  the  products 
of  combustion  are  carried  to  compartments  ahead  of  the  burn- 
ing compartments,  so  as  not  to  interfere  with  the  combustion 
in  the  second  burning  compartment.  The  arrangement  is  quite 
simple.  Assume  six  compartments — one  had  two  cooling,  three 
and  four  burning  and  five  and  six  heating  up.  The  hot  air 
from  compartment  one  is  carried  in  an  under-floor  blind  flue 
and  delivered  at  the  gas  ports  in  three,  and  similarly  the  hot 
air  from  two  is  delivered  to  four,  while  the  products  of  com- 
bustion from  three  and  four  are  delivered  respectively  into  five 
and  six.  A plan  is  also  worked  out  so  that  air  from  one  cool- 
ing compartment  may  be  divided  and  delivered  into  two  burning 
compartments  without  any  interference  of  the  combustion 
gases. 

A double  fired  kiln  is  developed  without  changing  the  gas 
delivery  of  the  Youngren  kiln,  by  simply  placing  the  gas  ports 
alternately  on  opposite  sides  of  the  division  wall,  with  bag 
walls  on  each  side.  The  under-floor  collecting  flues  are  ar- 
ranged to  deliver  air  to  the  several  bags. 

The  latest  arrangement  departs  materially  from  the  Young- 
ren kiln.  In  fact,  one  might  say  that  nothing  remains  except 
the  position  of  the  gas  port  and  the  method  of  delivering  the 
air  in  the  base  of  the  bag  below  the  gas  port.  This,  however, 
is  an  incontestible  feature  of  the  Youngren  kiln. 

The  gas  main  is  now  outside  the  kiln  wall  and  under  ground. 
A series  of  cross  flues,  corresponding  to  the  number  of  gas 
ports,  adjacent  to  and  parallel  with  the  sides  of  the  division 
walls,  underground,  in  fact,  below  the  level  of  the  under-floor 


286 


BURNING  CLAY  WARES. 


flues,  with  short  right  turns  to  the  centers  of  the  division  walls 
and  up-takes  to  the  gas  ports,  deliver  the  gas  to  the  ports. 
This  method  of  delivering  the  gas  to  the  kiln  is  used  in  Mend- 
heims  kiln,  Fig.  165,  in  the  Dunnachie  kiln,  Fig.  166,  and  as 
will  be  seen  in  the  Richardson  kiln. 

The  chief  difference  and  the  vital  factor  in  each  kiln  is  the 
method  of  getting  the  air  and  gas  together  at  the  point  of 
combustion.  One  may  consider  the  air  and  gas  port  as  a 
burner,  and  it  is  in  the  burner  that  the  several  kilns  men- 
tioned materially  differ.  In  fact,  the  validity  of  the  patents 
largely  depends  upon  the  burner. 

The  advantages  of  a double  fired  kiln  are:  (1)  Progressive 

equal  heating  of  the  division  walls  which  will  prevent,  or  meas- 
urably reduce,  the  drawing  over  tendency  illustrated  in  Fig. 
352.  (2)  It  insures  more  uniform  burns. 

The  Underwood  compartment  kiln  has  no  openings  through 


the  division  walls.  In  all  compartment  kilns  the  draft  flue  is 
centered  in  each  compartment  and  connects  with  a main  draft 
flue  outside  the  kiln  walls.  The  hot  air,  or  combustion  gas, 
is  collected  in  perforated  floor  flues  and  carried  forward 
through  the  division  walls,  thus  connecting  the  several  com- 
partments. The  Underwood  kiln  collects  the  air  or  gas  in  the 
usual  manner,  but  when  the  division  wall  is  reached  a flue 
parallel  to  the  wall  carries  the  air  outside  the  kiln  wall  into 
an  air  main,  and  it  is  brought  back  into  the  kiln  through  a 
similar  flue  on  the  opposite  side  of  the  wall  and  delivered 
through  ports  into  the  bases  of  the  bags.  The  main  gas  flu’s 
is  underground  outside  the  kiln  walls,  and  the  gas  is  deliv- 
ered under  the  kiln  through  a cross  flue  for  each  compartment 
in  front  of  the  bag  walls.  The  gas  and  air  delivery  flues  are 


Fig.  169. 


BURNING  CLAY  WARES. 


287 


superposed — the  former  being  underneath  and  from  it  the  gas 
is  conducted  to  the  gas  ports  in  the  division  walls,  as  shown 
in  Fig.  169. 

The  operation  of  the  air  flue  is  similar  to  that  of  an  ad- 
vanced heating  flue.  A compartment,  or  any  number  of  com- 
partments, may  be  by-passed  should  it  be  necessary  to  do  so 
for  repairs,  or  should  it  be  desirable  to  partially  isolate  a 
burning  compartment  for  reducing  conditions  or  salt  glazing. 
When  the  burning  has  advanced  to  the  point  where  reducing 


Fig.  171. 


conditions  are  desired,  or  for  salting,  the  air  supply  to  the 
burning  compartment  could  be  reduced  to  any  required  degree 
and  by  connecting  this  compartment  with  the  main  draft  flue 
and  disconnecting  the  air  escape,  the  operation  of  the  com- 
partment in  question  would,  except  in  the  air  supply,  be  inde- 
pendent of  the  other  compartments  without  in  any  way  inter- 
fering with  the  operation  of  the  latter. 

The  Richardson  kiln  is  a double  fired  compartment  kiln, 


288 


BURNING  CLAY  WARES. 


Fig.  172. 


Fig.  173. 


BURNING  CLAY  WARES. 


289 


shown  in  Fig.  170,  section,  Fig.  171,  plan,  Fig.  172,  section 
through  the  gas  connections,  and  Fig.  173,  sectional  perspec- 
tive. 

As  will  be  seen  from  the  plan  the  under-floor  system  con- 
sists of  alternate  open  and  covered  collecting  flues  with  the 
usual  central  draft  flue  in  each  compartment. 

The  collecting  flues  are  arranged  in  a staggered  manner 
— the  continuation  of  an  open  flue  in  one  compartment  becom- 
ing the  closed  flue  in  the  following  compartment.  Each  flue 
includes  two  compartments  within  its  length.  The  open  end 
collects  the  air,  and  the  covered  end  in  the  next  compartment 
delivers  the  air  to  the  combustion  bags.  The  combustion  gases 
are  similarly  collected  and  carried  to  the  heating-up  and  water- 
smoking compartments,  and  finally  are  drawn  off  through  the 
draft  flue  in  the  most  advanced  compartment.  Back  draft  is 
prevented  by  pasting  paper  to  form  a cover  on  top  of  the  bags 
in  the  compartment  ahead  of  the  draft  connection,  and  this 
method  of  dampering  continuous  kilns  is  common  practice  in 
both  the  compartment  and  the  tunnel  type  of  kilns. 

The  main  gas  flue  is  outside  the  kiln  wall  and  a series  of 
small  flues,  underground,  on  each  side  of  each  compartment, 
parallel  with  and  adjacent  to  the  division  walls,  deliver  the 
gas  into  the  bags  under  the  air.  Each  small  gas  flue  serves 
two  bags  (except  the  flues  supplying  the  end  bags)  and  each 
air  collecting  flue  likewise  serves  two  bags. 

It  has  long  been  held  that  we  cannot  get  reducing  conditions 
in  an  economizer  kiln.  In  such  kilns  we  get  complete  combus- 
tion and  use  a large  excess  of  air.  This  excess  occasions  no 
loss,  because  it  is  heated  by  the  cooling  ware  and  the  heat  is 
given  up  in  the  advanced  compartments  before  the  gases  are 
drawn  off  into  the  stack. 

Mr.  Richardson  has  succeeded  in  flashing  face  bricks  in  a 
very  simple  manner.  The  draft  in  the  burning  compartment 
is  checked,  which  cuts  down  the  volume  of  air  entering  this 
compartment.  The  gas  connections  into  the  succeeding  heat- 
ing-up compartment  are  opened  to  the  outside  air.  The  air 
thus  entering  this  compartment  not  only  further  checks  the 
draft  and  incoming  hot  air  in  the  burning  compartment,  but 
completes  the  combustion  of  the  unburned  gases  from  the 
compartment  under  reducing  conditions.  The  gas  comes  in 
under  its  own  pressure  and  with  the  air  supply  cut  off  or 
greatly  reduced,  the  burning  compartment  is  filled  with  un- 
burned gas,  thus  getting  the  reducing  conditions  necessary  for 
flashing. 


290 


BURNING  CLAY  WARES. 


The  unhurried  gas  passes  into  the  next  compartment  in 
advance,  where  it  comes  in  touch  with  the  air  entering  through 


the  producer  gas  ports,  its  combustion  completed,  and  the  tem- 
perature in  the  heating  up  compartment  is  correspondingly 
increased. 

The  gas  connection,  admission  and  distribution  is  shown 
in  Fig.  172. 


BURNING  CLAY  WARES. 


291 


The  Legg  design  is  illustrated  in  Fig.  174,  plan,  and  Fig. 
175,  sections.  In  several  of  the  compartment  kilns  one  can 
readily  trace  a development  from  an  earlier  kiln,  and  there 
is  a marked  similarity  in  the  general  arrangement  of  the 
several  flues  which  constitute  the  kiln,  and  in  the  operation 
of  the  kiln.  The  Legg  kiln  is  a distinct  departure  from  this 
general  plan. 

It  is  double  fired,  as  will  be  seen  from  the  plan,  and  the  gas 
is  introduced  from  the  outside  through  the  kiln  wall  into  the 
bag  space  on  each  side  of  each  compartment.  In  each  com- 
partment there  is  a longitudinal  draft  flue  as  usual.  The  hot 
air  for  combustion  is  drawn  down  through  a perforated  floor 
into  the  draft  flue  and  from  this  is  carried  forward  througn 
a flue  at  each  end  and  delivered  from  below  into  the  bag  space. 

The  producer  gas  enters  the  bag  spaces  horizontally  and  the 
air  rises  vertically  into  the  gas  and  both  are  carried  forward 


Fig.  175. 

into  the  bag  space  of  the  next  compartment.  The  top  of  the 
bag  space  is  crowned,  forming  an  enclosed  combustion  space, 
and  in  this  crown  are  a series  of  ports  for  the  escape  of  the 
burned  gases  into  the  compartments,  carrying  with  them  the 
heat  developed  by  the  combustion. 

The  division  walls  are  solid  above  the  floor  level,  and  also 
below  the  floor  level  except  the  two  hot  air  flues  above  men- 
tioned. 

The  plan  illustrated  is  semi-continuous  and  the  return  hot 
air  flue  is  centered  under  the  compartments,  returning  the  hot 
air  from  the  last  compartment  (on  the  right)  to  the  first  com- 
partment (on  the  left),  and  thence  it  is  distributed  through 
special  flues  to  the  four  comers  of  the  first  compartment. 

The  sketches  show  the  several  main  flues  underground  out- 
side the  kiln  wall — draft  (“D”),  gas  (“G’”),  and  advanced 


292 


BURNING  CLAY  WARES. 


heating  (“AH”) — but  the  actual  location  may  be  determined 
by  circumstances  and  be  varied  in  consequence.  Seemingly  it 
would  be  better  to  place  the  advanced  heating  flue  on  top  of 
the  kiln  or  in  the  upper  part  of  the  kiln  wall,  collecting  and 
delivering  the  hot  air  for  drying  through  hoods  from  man- 
holes in  the  kiln  crown,  in  the  usual  manner — at  least  this 
method  would  offer  less  resistance. 

The  plan  shown  is  for  producer  gas,  natural  gas,  or  oil, 
but  it  seems  to  the  author  that  gas  producer  furnaces  could 
be  attached  to  the  gas  openings  on  each  side  of  the  kiln  and 


Fig.  176. 

thus  get  direct  coal  firing  without  any  producer  loss  or  any 
requirement  in  steam  to  operate  the  producer.  Such  a plan 
was  worked  out  a number  of  years  ago,  and  theoretically,  so 
far  as  the  plan  was  developed,  no  difficulty  was  encountered. 

A section  through  one  of  the  division  walls  of  the  Goldner 
kiln  is  shown  in  Fig.  176.  The  bag  wall  is  arched  over  as  in 
the  Legg  design,  with  vents  in  the  top  corresponding  to  the 
feed  holes  in  the  kiln  crown.  The  hot  air  from  one  compart- 
ment is  collected  and  carried  forward  into  the  bag  wall  space 


BURNING  CLAY  WARES. 


203 


of  the  following  compartment  by  means  of  perforated  floor  and 
under  floor  flues. 

For  natural  gas  or  oil  firing  and  one  might  add,  powdered 
coal,  the  burner  is  inserted  in  the  feed  hole  and  the  downward 
current  of  gas  comes  in  contact  with  the  upward  current  of  air 
and  the  final  combustion  takes  place  under  the  kiln  crown. 

The  spandrell  flue — one  for  each  compartment — primarily 
is  for  producer  gas  supplied  from  a main  in  or  on  the  kiln 
wall  as  in  the  Youngren  kiln.  Damper  controlled  ports  in  the 
base  of  the  gas  flue  lead  the  gas  into  the  lowrer  part  of  the 
feed  hole,  thence  into  the  compartment  and  into  contact  with 
the  secondary  air.  This  spandrell  flue  also  serves  as  a col- 
lecting flue  for  advanced  heating.  The  connections  to  the  gas 
main  and  the  main  advanced  heating  flues  are  by  hoods  or 
goose-necks.  After  a compartment  is  burned  the  spandrell  flue 
is  cut  off  from  the  gas  flue,  and  after  the  cooling  has  progressed 
to  the  point  where  heat  may  safely  be  taken  for  water-smok- 
ing, connection  is  made  to  the  advanced  heating  flue  and  hot 
air  is  drawm  from  the  compartment  through  the  feed  hole  and 
gas  port  into  the  spandrell  flue,  thence  into  the  advanced  heat- 
ing flue  and  by-passed  to  the  compartments  ahead  of  the  burn- 
ing compartments  entering  the  latter  in  the  same  manner  as 
the  gas  is  introduced. 

The  sketch  shows  the  air  port  in  the  top  of  the  bag  as  being 
directly  under  the  feed  hole,  but  it  should  be  between  the  feed 
holes  and  alternate  throughout  the  length  of  the  compartment. 

Thus  a table  is  formed  under  the  feed  holes  for  coal  firing, 
or  for  salt  in  salt  glazing. 


294 


BURNING  CLAY  WARES. 


CHAPTER  XI. 


CAR  TUNNEL  KILN. 


HE  CAR  TUNNEL  KILN  idea  is  more  than  150  year3 


old,  but  the  impetus  to  the  development  of  the  kiln  came 


from  the  Bock  kiln,  presented  to  foreign  clayworkers 
about  forty-five  years  ago.  There  are  several  advantages  in  a 
car  tunnel  kiln  which  have  kept  the  idea  alive  in  spite  of  the 
numerous  early  failures.  These  may  be  enumerated  as  follows : 

(1)  The  economizer  principle,  with  its  marked  fuel  econ- 
omy. 

(2)  Centralization  of  the  fuel. 

(3)  A fixed  hot  zone  reducing  the  kiln  absorption  loss. 

(4)  Very  narrow  tunnels  which  are  easily  maintained,  and 
in  consequence  the  kiln  upkeep  is  relatively  slight. 

(5)  Only  the  limited  combustion  zone  requires  high  refrac- 
tory construction,  thus  lessening  the  initial  cost. 

(6)  Eliminating  two  handlings  of  the  ware;  namely,  the 
setting  and  the  drawing. 

There  are  some  offsets  to  these  advantages : 

(1)  The  fuel  economy  is  less  than  in  a large  tunnel  or 
compartment  economizer  kiln.  It  is  a question  of  the  relation 
of  the  mass  of  ware  to  the  kiln  mass,  in  which  the  large  kilns 
have  the  advantage. 

(2)  The  initial  cost  of  the  car  equipment  will  more  or 
less  even  up  the  difference  in  kiln  cost,  and  its  upkeep  will 
in  some  measure  counter-balance  the  greater  maintenance  of 
the  other  types  of  kilns. 

(3)  The  car  tunnel  kiln  has  not  yet  been  adapted  to  a 
wide  application.  The  failure  of  the  Bock  kiln,  and  two  or 
more  early  attempts  in  this  country,  was  due  to  the  water- 
smoking and  oxidation,  and  it  is  not  yet  proven  that  this  diffi- 
culty is  sufficiently  overcome  to  cover  a wide  range  of  clay 
materials. 

One  kiln  has  introduced  the  possibilities  of  an  advanced 


BURNING  CLAY  WARES. 


295 


heating  flue,  but  it  has  not  yet  been  tried  out.  We  are  of  the 
opinion  that  this  feature  of  the  kiln  must  come  if  we  are  to 
adapt  the  kilns  to  a number  of  common  wares.  One  can  readily 
see  that  if  a tender  clay  requires  one  to  several  days  to  water- 
smoke  and  only  a few  hours  to  burn,  or  relatively  long  periods 
for  oxidation  and  correspondingly  short  periods  for  burning 
we  must  either  hold  the  fire  on  the  burned  ware  an  unduly 
period  or  build  the  kiln  a prohibitive  length. 

Dressier  uses  two  tunnels — one  the  kiln  and  one  for  water- 
smoking and  heating  up — in  fact,  the  complete  plan  has  in  each 
unit  a third  tunnel  for  drying,  and  the  earlier  Bock  kiln  also 
had  two  tunnels.  American  practice  will  demand  that  the 
several  operations,  perhaps  exclusive  of  drying,  be  accom- 
plished in  a single  tunnel. 

There  are  upwards  of  fifty  car  tunnel  kilns  now  in  opera- 
tion in  this  country,  and  more  projected,  and  the  majority  of 
them  are  burning  pottery  wares.  In  several  instances  biscuit 
ware  is  burned  in  the  top  saggers  and  glost  ware  in  the  bottom 
saggers  in  order  to  adapt  the  ware  to  the  differences  in  kiln 
temperature.  Here  is  a point  that  becomes  very  important 
in  the  manufacture  of  certain  common  wares,  such  as  paving 
blocks,  face  bricks,  where  color  is  important,  and  products 
from  clays  having  very  short  burning  ranges. 

Products  Successfully  Burned. 

There  are  several  car  tunnel  kilns  operating  successfully 
on  fire  brick  products,  but  this  product  does  not  involve  the 
difficulties  in  drying  and  oxidizing  that  we  find  in  common 
clays,  nor  does  it  require  the  uniformity  of  temperature  that 
other  products  require.  There  are  two  kilns,  at  this  writing, 
operating  on  bricks  made  from  shale  which  may  or  may  not 
be  difficult  material  to  water-smoke  and  oxidize,  but  in  this 
product  we  have  the  common  clay  problems  and  the  success 
of  the  operation  will  be  a matter  of  deep  interest  to  clay- 
workers. 

When  the  car  tunnel  kiln  is  adapted  and  its  operation  ad- 
justed to  a ware,  there  is  undoubtedly  a balance  in  its  favor 
over  the  other  types  of  kilns. 

Early  Car  Tunnel  Kilns. 

The  Bock  car  tunnel  kiln  is  shown  in  Fig.  177,  longitudinal 
section,  Fig.  178,  plan,  and  Fig.  179,  cross  section. 

(Note:  These  drawings  and  all  the  car  tunnel  kiln  draw- 
ings are  not  to  scale,  being  shortened  without  proportionately 


296 


BURNING  CLAY  WARES. 


Figure  178 


BURNING  CLAY  WARES. 


297 


reducing  the  width  and  height  in  order  to  fully  illustrate  the 
principles.  Car  tunnel  kilns  vary  in  length  from  200  feet  to 
350  feet.  The  widths,  exclusive  of  any  side  wall  flues  or  spaces, 
range  from  four  feet  to  eight  feet  and  twelve  feet  is  claimed 
to  be  practical.  The  heights  above  the  car  decks  to  the  under 
side  of  the  crowns  are  around  five  feet.  It  may  also  be  noted 
that  in  the  sketches  no  attempt  is  made  to  show  the  details 
of  construction,  since  they  are  merely  intended  to  show  the 
principles  of  the  several  kilns.) 


A detailed  description  of  the  Bock  kiln  is  hardly  necessary. 
Briefly,  the  cars  are  covered  with  a refractory  floor  upon 
which  the  ware  is  placed.  An  apron  attached  to  the  car  floor 
or  to  the  top  of  the  car  frame  and  a sand-filled  trough  in  the 
kiln  wall  cut  off  the  metal  car  and  the  under  tunnel  from  the 
firing  tunnel.  This  is  the  ordinary  sand  seal  found  in  several 
kilns. 

The  air  supply  enters  under  the  cars  at  the  stack  end,  flows 


298 


BURNING  CLAY  WARES. 


the  length  of  the  kiln,  thus  keeping  the  cars  cool,  and  enters 
the  firing  tunnel  at  the  opposite  end,  thence  traveling  succes- 
sively through  the  cooling,  combustion,  and  heating-up  zones. 
A feature  of  the  kiln  which  has  been  applied  to  dryers  and  in 
modified  form  to  other  kilns,  is  the  metal  diaphragm,  forming 
the  inner  side  walls  at  the  receiving  (stack)  end.  The  purpose 
of  this  is  to  prevent  condensation  on  the  cold  wet  ware,  and 
at  the  same  time  recover  the  heat  from  the  waste  gases,  as 
well  as  the  latent  heat  from  the  condensing  water  vapor.  The 
diaphragm  flues  open  into  the  firing  tunnel  at  the  point  where 
the  waste  gases  are  assumed  to  become  fully  saturated,  and 
here  the  gases  are  removed  from  contact  with  the  ware,  but 
the  heat  from  them  passes  through  the  metal  plate  to  the  ware 
by  conduction  and  radiation. 

The  kiln  was  direct  coal  fired  through  the  feed  holes  in  the 
crown  of  the  firing  zone,  similar  to  a top-coal  fired  tunnel 
(ring)  kiln.  The  Siemens-Hesse  car  tunnel  kiln,  which  ap- 
peared three  or  four  years  after  the  Bock  kiln,  was  virtually 
a Bock  kiln  adapted  to  producer  gas. 

The  gas  was  introduced  into  the  combustion  zone  through 
the  inner  side  walls  virtually  in  the  same  manner  as  in  the 
present  day  kilns. 

It  is  interesting  to  trace  the  development  of  the  car  tunnel 
kiln.  The  Pechine  kiln  had  the  fire  at  one  end  and  one  can 
see  in  it  an  effort  to  develop  a tunnel  kiln  from  an  end  fired 
periodic  kiln  such  as  were  used  in  the  early  days. 

Borrie  moved  the  fires  to  the  center  of  the  tunnel.  Mean- 
while, or  later,  the  Hoffman  ring  kiln  is  developed,  and  Bock 
merely  converts  it  into  a car  tunnel  kiln,  in  a measure  com- 
bining the  Borrie  and  Hoffman  kilns. 

An  outline  of  the  Drayton  kiln  is  shown  in  Fig.  180,  longi- 
tudinal section,  Fig.  181,  plan  and  Fig.  182,  cross  section 
through  the  furnace.  The  kiln  is  in  reality  a compartment  car 
tunnel  kiln  with  advanced  heating  flue. 

(Note:  The  illustrations  do  not  show  the  full  number  of 
compartments.  A normal  kiln  will  have  four  compartments 
cooling,  one  burning,  three  heating  up,  and  two  water-smoking, 
corresponding  to  a ten-section  compartment  economizer  kiln.) 

The  kiln  is  divided  into  sections,  holding  three  cars  each, 
more  or  less  as  may  be  decided  in  designing  the  kiln.  Three 
cars  are  put  into  the  kiln  at  one  time  together  with  a short 
car  carrying  a division  wall  which  in  effect  is  equivalent  to 


(jr  tar.cpmtiiiition 


BURNING  CLAY  WARES. 


299 


300 


BURNING  CLAY  WARES. 


a division  wall  with  its  accompanying  bag  or  flash  wall  in  a 
compartment  economizer  kiln,  as  shown  in  Fig.  180. 

The  car  furniture  is  constructed  to  form  under  floor  flues 
which  connect  the  compartments  through  vertical  flues  in  the 
division  walls.  In  the  main  walls  on  each  side  of  the  division 
wall  cars  throughout  the  burning  and  heating-up  zones  are 
built  up  sliding  vertical  dampers,  and  through  a slot  in  the 
crown  of  each  division  wall  car  is  a horizontal  fire  clay  damper. 
When  three  cars  of  ware,  more  or  less  as  the  plan  may  be, 
followed  by  a division  wall  car,  are  put  into  the  kiln,  the 
division  wall  cars  come  opposite  the  damper  slots.  Exact 
placing  is  necessary  only  to  the  extent  that  the  damper  slots 


Figure  182. 

shall  come  within  the  limits  of  the  division  walls,  and  since 
the  division  walls  may  be  made  any  thickness,  ample  leeway 
may  be  provided  for  the  variation  in  the  position  of  the  cars. 
Following  each  lot  of  cars,  the  side  dampers  are  shoved  in  and 
the  top  dampers  lowered,  to  contact  with  the  division  walls. 
In  the  cooling  zone  up  to  the  combustion  zone  there  are  no 
dampers,  but  beyond,  each  zone  is  fully  dampered  by  vertical 
swinging  metal  dampers  in  the  side  walls  where  the  heat  does 
not  require  fire  clay  construction.  The  operation  is  as  follows : 
A fan  forces  air  into  the  cooling  zone,  through  the  ware, 
through  the  car  floor  flues,  and  under  the  cars,  up  to  the  com- 
bustion zone  division  wall.  The  air  under  the  cars  passes  for- 
ward and  is  delivered  under  the  furnace  grates.  The  com- 
bustion gases  pass  down  through  the  ware,  up  through  the 


BURNING  CLAY  WARES 


301 


302 


BURNING  CLAY  WARES. 


division  wall  flues,  into  the  next  compartment,  and  similarly 
through  several  compartments,  and  finally  are  drawn  off  by 
the  draft  fan. 

Hot  air  is  diverted  from  the  cooling  zone  through  a by- 
pass flue  to  the  water-smoking  compartments. 

The  advantages  which  the  kiln  offer  are : Independent  com- 
partment control,  down-draft  operation  and  separate  water- 
smoking. 

The  Faugeron  kiln,  illustrated  in  Fig.  183,  longitudinal 
section,  Fig.  184,  plan,  and  Fig.  185,  cross  section  through  the 
combustion  zone,  showing  air  inlets,  is  being  introduced  in  this 
country  in  the  manufacture  of  fire  bricks  and  pottery. 

The  kiln  is  divided  into  sections  by  drop  arches,  or  more 


Fig.  185. 


correctly  by  step  downs  in  the  tunnel  crown.  The  walls  are 
hollow  on  each  side  from  the  center  of  drop  arches  to  points 
midway  between  them,  and  the  inner  walls  are  perforated  at 
the  level  of  the  top  of  the  car  floor.  The  ware  on  the  car  is 
set  to  fit  the  tunnel  under  the  drop  arches  closely  so  that  in 
passing  these  points  the  free  area  is  so  restricted  that  the  air 
or  gases  will  be  at  least  in  a measure  forced  out  into  the  hol- 
low space  in  the  walls  to  return  to  the  tunnel  after  by-passing 
the  restricted  space.  One  will  readily  understand  the  theo- 
retical movement  of  the  air  and  gases.  The  tendency  always 
is  to  rise  into  the  crown  space  over  the  cars  and  to  move 
forward  under  the  influence  of  the  draft.  When  the  air  reaches 
a drop  arch  it  is  forced  downward,  and  since  it  cannot  readily 


BURNING  CLAY  WARES. 


303 


pass  forward  under  the  drop  arch,  nor  alongside  the  cars,  it 
must  pass  down  among  the  ware  out  through  the  wall  perfora- 
tions into  the  hollow  space,  and  then  after  passing  the  drop 
arch  section,  it  is  forced  to  re-enter  the  tunnel  at  the  car 
floor  level  and  naturally  rises  into  the  crown  space.  This 
sinuous  course,  even  though  the  operation  is  imperfect,  suffices 
to  bring  the  air,  or  gases,  fully  in  contact  with  the  ware.  In 
the  combustion  zone,  direct  coal  fired,  though  not  necessarily 
so,  the  hot  air  is  distributed  under  the  grate  bars  for  primary 
combustion  and  through  the  bridge  and  diaphragm  walls  for 
secondary  combustion.  The  combustion  gases  enter  the  tunnel 
through  the  diaphragm  wall  at  the  car  floor  level  and  thence 
take  a sinuous  course  to  the  draft  outlet,  thus  giving  up  to 
the  green  ware  the  waste  heat  just  as  the  entering  air  collects 
the  heat  from  the  cooling  ware.  It  may  be  noted  that  the 
ware  must  be  of  such  a character  and  so  set  as  to  offer  greater 
resistance  to  the  passage  of  the  air  and  gas  under  the  drop 
arches  than  through  the  wall  slots  and  by-pass  flues. 

A heat  balance  of  such  a kiln  by  Prof.  C.  B.  Harrop  in 
Vol.  XIX,  Trans.  American  Ceramic  Society,  shows  the  fol- 
lowing results : 

3.27%  Unconsumed  combustible  matter  in  the  ash  pits. 

35.83%  stack  losses,  including  dehydration. 

17.27%  in  cars  and  ware  leaving  the  kiln. 

45.00%  radiation  loss. 

101.37%  total. 

The  results  show  greater  consumption  than  fuel  supply,  but 
considering  that  all  the  determination,  including  radiation, 
were  direct,  the  results  check  up  very  closely  and  are  more 
satisfactory  than  data  in  which  some  important  item  is  deter- 
mined by  difference.  The  usual  method  is  to  determine  radia- 
tion and  kiln  losses  by  difference  and  thus  hide  the  errors 
which  may  have  been  made  in  the  direct  determination  of  the 
other  factors. 

The  heat  balance  of  a producer  gas  fired  economizer  kiln  by 
Prof.  R.  K.  Hursh,  University  of  Illinois,  in  Yol.  I,  No.  S, 
Journal  American  Ceramic  Society,  is  as  follows: 

.6%  unconsumed  combustible  matter  in  ash. 

14.7%  producer  loss. 

32.4%  stack  loss. 

43.6%  consumed  by  the  ware. 

8.7%  radiation  and  kiln  loss. 


304 


BURNING  CLAY  WARES. 


It  is  to  be  regretted  that  scientists  have  not  adopted  some 
definite  interpretations  of  heat  balances,  and  it  is  also  to  be 
regretted  that  the  results  are  not  given  in  pounds  of  fuel. 

A.  V.  Bleininger,  in  Heat  Balances  of  Industrial  Kilns,  gave 
the  percentage  fuel  consumptions  required  for  the  ware  as 
follows : 

Sewer  pipe,  5.7  per  cent. ; paving  bricks,  11.3  per  cent. ; terra 
cotta,  12.6  per  cent,  and  8 per  cent.;  common  bricks,  19.6  per 
cent. 

Prof.  Harrop,  it  will  be  noted,  does  not  give  any  heat  re- 
quired by  the  ware  except  that  removed  from  the  kiln  in  the 
cooling  bricks  and  the  dehydration  losses. 

It  is  beyond  our  present  knowledge  to  estimate  the  heat 
actually  required  to  burn  clay  wares.  We  must  heat  the  ware 
to  a certain  temperature,  and  during  this  heating  certain  pyro- 
chemical  changes  take  place  which  may  or  may  not  require 
heat,  but  probably  very  little  heat  is  required  to  effect  these 
changes  or  given  up  by  them.  The  estimated  heat  required 
then  is  based  on  the  temperature  and  specific  heat  of  the  clay 
and  ware,  taking  into  consideration,  of  course,  the  heat  re- 
quired for  dehydration. 

The  heat  required  for  any  given  ware  will  be  the  same  in 
any  type  of  kiln  and  the  economy  of  the  kiln  will  depend  upon 
the  other  factors,  particularly  the  possibility  of  recovering  the 
heat  in  the  ware.  In  Bleininger’s  kilns  there  was  no  recovery 
of  the  heat  from  the  ware,  at  least  not  for  use  in  the  kilns, 
and  when  the  burns  were  finished  the  ware  held  the  heat  value 
given  in  the  percentages. 

The  percentages  are  low  simply  because  the  total  fuel  re- 
quired for  the  operations  was  high. 

In  two  of  the  five  periodic  kilns  the  radiation  loss  in  per 
cent,  was  less  than  Harrop  found  for  the  car  tunnel  kiln,  but 
in  pounds  of  fuel  the  periodic  kilns  will  show  a higher  loss 
than  the  car  tunnel  kiln. 

Prof.  Hursh,  in  the  compartment  economizer  kiln  test,  gives 
the  ware  requirement  at  43.6  per  cent.,  which  considering  the 
total  amount  of  fuel  used  and  the  kiln  temperatures  attained, 
will  compare  approximately  with  Bleininger’s  results,  but  all 
of  this  heat  is  given  back  to  the  kiln  operation  and  the  heat 
actually  retained  by  the  ware  is  practically  none.  In  conse- 
quence of  setting  aside  43.6  per  cent,  of  the  total  fuel  for  the 
ware,  Prof.  Hursh  gets  by  difference  the  remarkably  low  radia- 
tion loss  of  8.7  per  cent.  He  has  not  taken  into  account  the 


BURNING  CLAY  WARES. 


305 


heat  from  the  cooling  ware  which  is  carried  forward  into  the 
burning  compartments  and  the  actual  heat  lost  by  radiation 
during  the  period  of  the  test  will  be  that  given,  plus  the  heat 
derived  from  the  cooling  ware. 

Clayworkers  are  deeply  interested  in  the  study  of  kilns, 
and  it  is  unfortunate  that  any  technical  data  should  be  mislead- 
ing. On  the  face  of  the  returns  from  the  several  tests  the  clay 
worker  would  naturally  give  the  preference  to  an  economizer 
kiln  in  which  the  radiation  loss  is  only  8.7  per  cent,  of  the 
remarkably  low  total  fuel  consumption.  In  pounds  of  coal  this 
loss  would  be  very  small.  Compared  with  this,  the  45  per  cent, 
radiation  loss  from  Prof.  Harrop’s  car  tunnel  kiln  with  its 
higher  total  fuel  consumption,  makes  a poor  showing. 

The  car  tunnel  kiln  may  be  less  economical  in  fuel  than  the 
compartment  or  other  type  of  economizer  kiln,  but  there  is  no 
such  itemized  difference  as  the  two  heat  balances  show,  and 
there  would.be  still  less  difference  were  the  same  ware  being 
burned. 

Both  kilns  can  reduce  the  temperature  of  the  gases  to  a 
minimum  before  they  are  drawn  off  into  the  stack  and  in  stack 
losses  on  the  same  ware  there  should  be  little  difference.  Both 
types  can  be  thoroughly  insulated  and  the  radiation  loss  should 
not  widely  vary,  while  in  the  ground  and  kiln  absorption  loss 
the  car  tunnel  has  the  preference  because  the  hot  zone  is  fixed 
and  there  is  no  cold  wall  mass  to  be  heated  up  as  in  the  com- 
partment kiln. 

The  ware  from  each,  if  the  kilns  are  properly  designed, 
will  approximate  atmospheric  temperature  before  it  leaves 
the  kiln,  and  the  heat  actually  consumed  by  the  ware  will  be 
the  same  in  either  type  of  kiln. 

The  car  tunnel  kiln  tested  by  Prof.  Harrop  is  197  feet  long, 
4 feet  4 inches  wide  inside  the  tunnel,  and  8 feet  high  from 
the  rail  to  the  under  side  of  the  crown  in  the  center. 

The  output  in  fire  bricks  is  about  30  tons  per  day  burned 
to  cone  8.  A subsequent  report  by  another  authority  shows 
temperatures  of  cone  13  to  cone  14,  which  are  the  tempera- 
tures claimed  for  the  kiln  in  question. 

The  coal  consumption  during  the  test  by  Prof.  Harrop  was 
226  pounds  of  coal  per  ton  of  ware,  the  heat  value  of  the  coal 
being  13815  B.t.u.  The  other  authority  mentioned  reported 
250  pounds  of  coal  per  ton  of  ware  burned  to  cone  14. 

These  results  are  not  comparable  with  the  results  given 
for  the  compartment  kiln  by  Prof.  Hursh,  nor  would  they  be  if 


306 


BURNING  CLAY  WARES, 


BURNING  CLAY  WARES. 


307 


the  latter  had  distributed  his  results  as  did  Prof.  Harrop  and 
given  them  in  pounds  of  coal,  simply  because  the  wares  are 
different  and  the  temperatures  attained  are  widely  apart. 

It  is  really  immaterial  what  method  of  distributing  the  data 
is  adopted,  but  there  should  be  a standard  method.  We  would 
prefer  to  have  the  results  in  pounds  of  fuel  and  placed  where 
they  belong,  but  since  the  fuel  required  to  heat  the  ware  is 
approximately  the  same  for  each  cone  temperature  the  per- 
centage attributed  to  it  becomes  a comparative  measure  of 
the  efficiency  of  the  kiln.  For  instance,  in  Prof.  Hursh’s  heat 
balance  the  kiln  efficiency  ranks  high  because  43.6  per  cent,  of 
the  total  fuel  is  attributed  to  the  ware,  while  the  kilns  tested 
by  Mr.  Bleininger  would  rank  low  since  only  from  5 per  cent, 
to  20  per  cent,  of  the  total  fuel  is  attributed  to  the  ware.  In 
other  words,  the  latter  kilns,  if  the  ware  had  been  the  same, 
would  require  from  two  to  nine  times  as  much  fuel  per  ton  of 
ware  as  the  former. 

The  Dressier  kiln  is  illustrated  in  Fig.  186,  a diagrammatic 
plan,  Fig.  187,  a cross  section  through  the  combustion  zone, 
and  Fig.  188,  cross  sections  through  the  heating  up  and  cool- 
ing zones. 

The  heating  up  zones  varies  in  length  from  50  feet  to  120 
feet  and  may  be  longer  if  desired.  It  is  in  this  zone  that  dehy- 
dration and  oxidation  take  place,  and  the  successful  accom- 
plishment of  these  operations  is  an  important  factor  in  the 
wide  application  of  the  kiln.  Pottery  and  sanitary  wares  are 
being  burned  successfully  and  economically  in  this  kiln,  and 
also  fire  clay  products,  but  in  these  wares  the  oxidation  diffi- 
culties are  a minimum.  Some  of  the  common  clay  wares  are 
exceedingly  difficult  to  oxidize.  The  heat  in  the  gases  from 
the  combustion  zone  is  recovered  by  conduction,  radiation,  and 
convection  in  the  heating  up  zone.  These  gases  do  not  come  in 
contact  with  the  ware,  but  instead  are  carried  through  the 
piping  shown  in  the  cross  section  of  the  heating  up  zone. 

The  volume  of  air  in  the  heating  up  end  of  the  tunnel  is 
very  small,  being  that  introduced  by  leakage,  but  any  volume 
can  readily  be  introduced  and  this,  it  is  said,  may  be  carried 
forward  to  any  desired  point  and  there  drawn  off  and  by- 
passed to  the  furnace  bench. 

This  would  provide  the  air  essential  to  thorough  oxidation. 

The  rate  of  oxidation  is  determined  by  the  drop  in  tem- 
perature of  the  combustion  gases.  If  the  oxidation  is  too 
rapid,  we  might  move  the  cars  forward  at  a slower  rate,  but 


308 


BURNING  CLAY  WARES. 


this  would  involve  holding  the  ware  longer  in  the  combustion 
zone,  which  would  not  be  desirable  if  for  no  other  reason  than 
the  reduction  in  capacity  without  corresponding  reduction  in 
fuel  consumption.  The  volume  of  gases  and  consequent  heat 
supply  cannot  be  changed  because  this  depends  upon  the  com- 
bustion requirement,  but  the  drop  in  temperature  requisite  for 
any  oxidation  can  be  adjusted  (in  a longer  tunnel)  by  the 
character,  number,  size,  and  shape  of  the  waste  gas  flues  and 
piping.  Longer  stretches  of  fire  clay  ducts  with  thicker  walls 
with  a single  large  iron  pipe  extension  to  the  draft  outlet  in- 
stead of  the  several  pipes  shown,  will  lengthen  the  heating  up 
zone  and  result  in  a more  gradual  drop  in  temperature  as  may 
be  required. 

In  the  ordinary  economizer  kiln  the  combustion  gases  pass 
forward  into  the  compartments  of  green  ware,  or  at  least  into 
the  compartments  in  which  the  ware  is  being  heated  up  and 
oxidized,  and  the  ware  is  subjected  to  the  ill  effects  of  the 
combustion  gases,  particularly  sulphur,  including  the  sulphur 
from  the  ware  which  often  exceeds  that  from  the  fuel.  The 
bad  effects  of  sulphur  are  especially  severe  where  the  kiln  is 
not  equipped  with  an  advanced  heating  flue. 

The  operation  of  the  Dressier  car  tunnel  kiln  differs  ma- 
terially. 

The  air  for  combustion  enters  at  the  delivery  end  and 
passes  through  the  tunnel  in  contact  with  the  cooling  ware, 
up  to  the  combustion  zone.  In  this  zone,  on  each  side,  is  a 
combustion  duct  (furnace  bench)  built  of  fire  bricks  and  lined 
with  carborundum.  The  gas,  if  from  a producer,  is  delivered 
to  the  kiln  through  a cross  duct  under  the  kiln  base,  and  rises 
through  vertical  ports  into  the  combustion  ducts.  The  air  is 
drawn  into  the  combustion  ducts  through  ports  in  the  furnace 
bench  and  enters  the  ducts  behind  the  gas,  although  there  is, 
or  may  be,  air  admission  for  secondary  combustion  in  front 
of  the  gas  port,  or  for  a second  gas  port.  In  fact,  the  relative 
positions  of  the  air  and  gas  entries  may  be  anything  the  de- 
signer desires.  The  combustion  ducts  connect  directly  with 
the  carborundum  double  walled  combustion  chambers  shown 
in  Fig.  187.  The  central  ducts  in  these  chambers  carry  the 
combustion  gases,  while  the  surrounding  transverse  flues  are 
for  air  circulation  and  heat  convection  in  the  heating  up  zone. 
Iron  pipes  extend  the  combustion  ducts  to  the  draft  outlet. 

It  is  seen  that  the  air  enters  in  the  cooling  end,  is  heated 
by  the  cooling  ware,  enters  the  combustion  ducts  near  the 


BURNING  CLAY  WARES. 


309 


Fig.  187. 


Fig.  188. 


310 


BURNING  CLAY  WARES. 


longitudinal  center  of  the  kiln,  and  the  combustion  gases  are 
drawn  off  through  the  heating  up  end  by  means  of  ducts  and 
pipes.  It  is  evident  that  no  air  except  that  from  leakage  enters 
the  heating  up  end  of  the  tunnel,  although  air  in  greater  quan- 
tity, if  desired  for  oxidation,  may  be  introduced  through  this 
end.  Any  gas  or  vapor  from  the  ware  in  the  heating  up  zone 
cannot  pass  back  over  the  cold,  possibly  wet,  entering  ware, 
but  instead  must  move  forward  to  the  combustion  benches, 
where  it  commingles  with  the  hot  air  from  the  cooling  end  of 
the  tunnel  unless  it  be  drawn  off  at  some  intermediate  point. 

In  the  heating  up  zone  are  the  hot  waste  gas  ducts  and 
pipes,  and  any  air  in  the  tunnel  will  tend  to  rise  through  the 
circulating  channels  and  among  the  pipes  into  the  tunnel  crown 
space  and  then  drop  through  the  cooler  ware.  Since  the  air 
is  replaced  by  leakage  to  some  extent  and  in  larger  volume 
intermittently  when  the  end  doors  are  opened  to  introduce  cars 
of  green  ware,  there  will  be  in  conjunction  with  the  transverse 
circulation,  a forward  movement,  or  in  other  words,  theoreti- 
cally, there  will  be  a slow  spiral  forward  movement  of  the  air 
through  the  heating  up  tunnel,  which  may  be  materially  in- 
creased if  desired. 

The  cooling  zone  has  iron  pipes  on  each  side,  open  to  the 
outside  in  the  end  wall,  and  connected  with  a fan  at  the  end 
near  the  combustion  zone.  Through  these,  air  is  drawn  and 
heated  for  use  in  independent  dryers  or  for  other  purposes 
in  the  factory. 

A baffle  wall  is  placed  between  the  air  pipes  and  the  hot 
ware  to  develop  the  same  circulating  conditions  as  in  the  other 
end  of  the  tunnel ; namely,  a transverse  circulation  of  the  air 
in  conjunction  with  its  forward  movement  in  the  tunnel,  and 
the  volume  of  air  is  that  required  for  combustion,  together 
with  more  or  less  excess. 

It  is  of  first  importance,  of  course,  to  conserve  the  heat  in 
the  cooling  ware  for  the  kiln  operation,  but  any  excess  can  be 
utilized  for  other  work  with  the  added  advantage  of  getting 
the  ware  cooled,  which  is  not  always  the  case  where  the  cool- 
ing end  of  the  tunnel  is  limited  in  length. 

When  the  kiln  temperature  to  be  attained  is  not  excessive 
a larger  volume  of  heat  may  be  drawn  off  for  other  work,  even 
to  the  extent,  if  desired,  that  the  heat  so  removed  must  be 
replaced  in  the  kiln  by  the  consumption  of  increased  quan- 
tity of  gas.  It  is  simply  a question  of  whether  it  is  more 
economical  to  generate  heat  for  outside  purposes  in  the  kiln 


BURNING  CLAY  WARES. 


311 


or  independently.  In  this  instance  the  kiln  has  the  prefer- 
ence most  decidedly. 

Where  high  temperatures  are  required  in  the  kiln  the  first 
consideration  must  be  conservation  of  heat  from  the  cooling 
ware  to  augment  the  heat  from  combustion,  and  heat  for  out- 
side work  becomes  secondary. 

Dressler’s  idea  in  its  completeness  has  three  tunnels.  One 
is  the  kiln  proper.  The  second  is  a pre-heater  and  the  heat 
is  supplied  by  the  hot  air  and  by  the  products  of  combustion 
from  the  firing  tunnel,  the  former  introduced  into  the  second 
tunnel  direct  and  the  latter  drawn  through  wall  pipes. 

The  temperature  in  the  pre-heater  is  said  to  be  100  deg.  C. 
to  250  deg.  C.  The  air  from  the  pre-heating  tunnel  is  drawn 
through  side  pipes  in  a third,  or  drying  tunnel,  and  not  only 
is  the  available  sensible  heat  recovered,  but  the  vapor  from 
the  ware  in  the  pre-heater,  which  represents  a large  part  of 
the  heat,  is  condensed  in  the  dryer  tunnel  pipes  and  its  latent 
heat  given  up. 

The  data  relative  to  the  fuel  consumption  is  varied,  but  does 
not  give  us  a fair  comparison  with  that  of  other  types  of  kilns. 

We  get  the  following  data  from  a paper  by  A.  Bigot  in 
Vol.  XV,  Transactions  English  Ceramic  Society. 

Percentage  of  fuel  value  attributed  to  the  ware  in  three 
types  of  kilns: 


The  Hoffman  kiln,  it  will  be  understood,  is  the  ordinary 
tunnel,  or  ring  kiln,  and  the  periodic  is  presumably  a down- 
draft.  If  the  above  data  is  correct,  then  a unit  of  fuel  will 
burn  one  ton  of  ware  in  the  periodic  kiln,  a ton  and  one-half 
in  the  Hoffman,  and  more  than  three  tons  in  the  Dressier. 

The  data  is  interesting,  but  not  acceptable.  We  should 
know  whether  the  ware  was  the  same,  and  also  we  should 
know  the  size  and  character  of  the  periodic  kiln.  On  the 
same  yard  we  have  found  down-draft  kilns  using  two  and  more 
times  as  much  fuel  per  ton  of  ware  than  was  being  used  in 
other  kilns,  simply  due  to  the  difference  in  size  of  the  kiln. 

Mr.  C.  J.  Kirk,  in  Yol.  XVIII,  Transactions  American  Cera- 
mic Society,  states  that  15,000  pounds  of  sanitary  ware  are 
being  burned  daily  with  25,000  cu.  ft.  of  natural  gas,  whereas 


Dressier 

Hoffman 

Periodic 


49.6  per  cent. 
22.9  per  cent. 
15.0  per  cent. 


312 


BURNING  CLAY  WARES. 


BURNING  CLAY  WARES. 


313 


the  same  weight  of  ware  formerly  burned  in  periodic  kilns 
required  250,000  to  300,000  cu.  ft.  of  natural  gas. 

These  figures  are  startling  and  cannot  be  accepted  without 
some  explanation  of  the  gas  firing  in  the  periodic  kilns.  Nat- 
tural  gas  is  a very  difficult  fuel,  economy  considered,  in  a 
periodic  kiln.  The  flame  is  short  and  very  intense.  It  is  neces- 
sary to  protect  the  furnace  brick  work  and  to  get  the  heat  over 
into  the  kiln,  to  introduce  an  excessive  quantity  of  air  through 
the  furnaces  to  sweep  the  heat  out  of  the  furnace.  The  stack 
losses  under  such  circumstances  are  excessive.  It  is  possible 
to  fire  a kiln  indefintely  without  getting  sufficient  heat  from 
the  furnaces  into  the  kiln  through  lack  of  excess  that  the  gas 
combustion  temperature  is  never  high  enough  to  burn  the 
ware. 

A pottery  manufacturer  puts  the  difference  in  gas  consump- 
tion in  the  two  types  of  kilns  in  a single  sentence ; namely, 
“Gas  bills  for  periodic  kiln  operation  were  $3,000.00  per  month, 
and  now  for  Dressier  kiln  operation  they  are  $1,000.00  per 
month.” 

* 

This  is  a saving  of  66  per  cent.,  and  it  may  be  accepted  as 
practical.  Ordinary  tunnel  and  compartment  kilns  show  a 
saving  of  50  per  cent,  to  75  per  cent,  over  the  periodic  kiln, 
and  the  car  tunnel  kilns  have  the  same  regenerative  principles 
as  the  ordinary  continuous  kilns  and  should  show  similar  sav- 
ing over  the  periodic  kiln. 

A Dressier  kiln  is  to  be  installed  in  an  Ohio  factory  to  burn 
common  bricks,  hollow  bricks  and  drain  tile.  The  kiln  will  be 
about  300  feet  long,  and  it  is  expected  to  turn  out  approxi- 
mately 100  tons  of  hollow  ware  per  day.  The  cars  are  to  be 
6 feet  by  8 feet,  and  the  setting  will  be  4 feet  high,  perhaps 
5 feet.  It  is  expected  to  pull  the  cars  at  the  rate  of  one  every 
forty-five  minutes.  The  operation  will  be  continuous  and  extra 
cars  will  be  provided  to  maintain  the  operation  of  the  kiln 
throughout  the  night  and  over  Sunday. 

The  manufacturers  of  common  clay  wares  will  watch  the 
result  of  this  operation  with  a good  deal  of  interest.  Up  to 
date  there  have  been  several  attempts  to  burn  such  common 
wares  in  car  tunnel  kilns,  but  without  satisfactory  results. 

The  Zwermann  single  tunnel  kiln  is  illustrated  in  Fig.  189, 
longitudinal  section,  Fig.  190,  a diagrammatic  plan,  and  Fig. 
191,  a cross  section  through  the  burning  zone. 

The  sketches  are  broken  to  reduce  them  to  a suitable  length 
for  illustration,  but  the  furnace  section  is  shown  full  length. 


314 


BURNING  CLAY  WARES. 


In  the  cooling  end  are  shown  flues  in  the  side  wall,  or 
rather  forming  a part  of  the  side  wall,  and  also  it  will  be 
noted  that  the  tunnel  crown  is  double,  thus  forming  a crown 
flue.  The  lower  flues  in  each  side  and  the  crown  flue  have  no 
part  in  the  kiln  operation  except  cooling. 

These  flues,  which  are  one  hundred  feet  long,  more  or  less, 
are  open  to  the  outside  air  at  the  delivery  end  of  the  kiln,  and 
near  the  furnace  section  they  terminate  in  a fan  which  draws 
the  air  through  these  flues  and  delivers  it  into  a dryer  or  to 
other  purposes  in  the  factory  independent  of  the  kiln. 

The  other  flues  in  the  side  walls,  one  on  each  side  of  the 
tunnel,  have  a pressure  fan  connection  at  the  delivery  end  of 


Fig.  191. 

the  kiln,  which  forces  air  through  the  flues  and  delivers  it  to 
the  furnace  for  combustion. 

The  combustion  gases  are  drawn  off  through  a fan  near 
the  receiving  end  of  the  kiln. 

The  dimensions  of  the  kiln  are,  pre-heating  zone  approxi- 
mately 150  feet  long,  furnace  zone  nearly  60  feet  and  cooling 
zone  about  140  feet.  The  tunnel  is  6 feet  4 inches  wide  and 
6 feet  6 inches  high  above  the  trucks.  The  lengths  of  the  sev- 
eral zones  and  their  total  length  will  vary  in  different  kilns, 
depending  upon  the  character  of  the  work  to  be  done. 

The  furnaces  are  staggered  except  the  final  two  burners, 
and  it  would  seem  that  the  purpose  of  this  staggered  arrange^ 
ment  is  to  get  the  equivalent  of  cross  firing  in  the  arches  of 
up-draft  kilns.  The  furnace  box  or  arch  is  built  on  the  truck 


BURNING  CLAY  WARES. 


315 


transversely — one  centered  on  each  truck — and  the  ware  is 
piled  on  each  side  and  over  the  arch.  In  this  we  plainly  have 
the  idea  of  the  arches  in  an  up-draft  field  kiln.  If  the  ware 
being  burned  in  the  tunnel  kiln  were  bricks  the  setting  would  be 
such  as  to  form  the  combustion  arches  from  the  bricks  as  in 
a scove  kiln.  As  the  cars  advance  the  arches  come  opposite, 
first  one  furnace  on  one  side,  then  the  next  furnace  on  the 
opposite  side,  repeating  to  the  end  of  the  furnace  section  where 
the  finishing  touches  to  the  ware  are  given  by  a pair  of  oppo- 
site furnaces. 

A later  design  by  Zwermann,  shown  in  Fig.  192,  cross  sec- 


tions, and  Fig.  193,  plan,  has  a double  tunnel,  operating  in 
opposite  directions.  At  either  end  there  will  be  green  ware 
going  into  one  tunnel  and  burned  ware  coming  out  of  the 
other  tunnel.  Near  the  end  where  water-smoking  takes  place 
in  one  tunnel  and  cooling  in  the  other  the  division  wall  be- 
tween the  two  tunnels  is  thin,  in  fact,  toward  the  end  where 
the  temperature  of  the  cooling  ware  is  a low  red  heat  or  less, 
the  division  wall  is  steel  plate  stiffened  by  pilaster  walls,  which 
also  support  the  crowns.  On  one  side  of  this  wall  we  have 
cooling  ware  coming  out  and  on  the  other  side  green  ware 
going  in,  with  the  reverse  at  the  opposite  end  of  the  kiln. 

By  convection  the  heat  from  the  cooling  ware  is  brought 
into  contact  with  the  thin  division  wall  and  by  conduction  it 
is  carried  through  this  wall  into  the  adjacent  tunnel  where 


316 


BURNING  CLAY  WARES. 


baffles  direct  the  currents  of  air  and  take  the  heat  from  the 
wall  to  the  green  ware. 

The  advancing  green  ware  next  passes  into  a muffled  sec- 
tion of  the  tunnel.  The  combustion  gases  from  the  furnace 
zone  are  drawn  back  through  tubes  and  the  forward  moving 
ware  is  heated  up  by  the  air  in  the  tunnel  circulating  among 
these  waste  gas  ducts  and  taking  the  heat  therefrom  to  the 
ware. 

This  muffled  section  is  the  heating-up  zone.  From  the 
muffled  zone  the  ware  passes  the  furnaces,  where  it  comes 
in  direct  contact  with  the  flame  from  the  furnaces  or  burners 
and  the  combustion  gases  pass  down  among  the  ware  to  the 


Fig.  194. 


car  floor  level,  thence  back  to  the  draft  flue  in  the  side  wall 
and  forward  into  the  heating-up  ducts  above  mentioned  and 
out  through  the  stack. 

The  ware  goes  forward  into  the  cooling  section,  where  the 
heat  is  given  up  to  the  ware  coming  in  through  the  other 
tunnel.  The  two  tunnels  are  identically  alike  except  the  sev- 
eral zones  are  reversed. 

Along  the  central  portion  of  the  kiln  the  crown  is  double 
with  a space  between,  and  the  air  for  secondary  combustion 
is  drawn  through  this  annular  space  and  becomes  heated  be- 
fore bringing  it  into  touch  with  the  gas. 

Another  car  tunnel  kiln  which  is  finding  use  in  this  country 
is  illustrated  in  Fig.  194,  a cross  section  through  the  cooling 
zone  looking  toward  the  furnace  zone.  The  kiln  is  very  sim- 
ple. It  has  a straight  tunnel  except  an  enlarged  section  for 
the  furnaces.  There  is  no  sand  seal,  but  instead  the  car  fur- 


BURNING  CLAY  WARES. 


317 


niture  recesses  loosely  into  a groove  in  the  side  walls.  The 
furnace  section  is  simply  a wider  span  overlapping  the  tunnel 
width,  which  is  boxed  in  on  sides  and  ends  to  form  a combus- 
tion box  on  each  side  of  the  kiln.  The  fires  are  in  the  ends 
of  this  box  and  the  flame  is  parallel  with  the  moving  cars  until 
deflected  by  the  furnace  crown.  In  several  installations  there 
are  no  flash  or  perforated  walls  between  the  firebox  and  the 
tunnel.  In  fact,  it  is  said  that  the  curved  crown  of  the  furnace 
section  reflects  the  heat  to  the  ware  to  better  purpose  than  is 
possible  through  direction  by  means  of  flash  walls. 

Under  the  hot  sections  of  the  kiln  and  underneath  the  cars 
is  a longitudinal  open  duct  which  is  connected  with  the  out- 
side air  by  under  cross  ducts  with  vertical  risers  to  the  ground 
level  outside  the  kiln  wall.  The  connection  between  the  cross 
duct  and  the  longitudinal  duct  is  dampered,  and  the  only  air 
normally  entering  the  longitudinal  duct  is  by  leakage,  but  this 
can  be  increased  to  any  degree  by  opening  the  damper  connec- 
tion. The  purpose  of  these  ducts  is  to  supply  sufficient  air  to 
protect  the  cars,  and  this  air,  since  there  is  no  sand  seal,  rises 
into  the  tunnel  proper.  It  is,  therefore,  desirable  that  the 
volume  of  air  entering  in  this  way  should  be  kept  to  a mini- 
mum, and  this  will  depend  upon  the  amount  required  to  keep 
the  cars  cool. 

The  walls  of  the  cooling  end  of  the  kiln  are  of  a hollow 
construction  and  the  vertical  flues  formed  within  the  walls  by 
this  construction  are  open  to  the  air  on  top,  and  likewise  at 
the  bottom  through  ports  in  the  outside  wall.  The  outside 
air  thus  can  freely  enter  these  wall  flues  at  the  bottom,  and 
naturally  rising,  it  escapes  through  the  top  ports. 

The  inwall  is  made  thin  and  the  heat  from  the  cooling 
ware  is  conducted  through  the  wall,  picked  up  by  the  air 
currents  in  the  wall  flues  and  removed  from  the  kiln.  No  use 
is  made  of  this  waste  heat  except  as  it  serves  to  heat  the  fac- 
tory in  cold  weather,  but  of  course  it  could  be  readily  collected 
and  applied  to  any  purpose  in  the  factory  operation,  or  be 
carried  forward  and  used  in  the  furnaces.  In  one  plan  pro- 
vision is  made  for  the  use  of  the  air  in  the  combustion  by  sim- 
ply collecting  it  by  down-draft  into  a flue  and  carrying  it  for- 
ward to  the  furnace.  This  provision,  however,  has  not  been 
installed  in  the  kiln  in  question. 

In  the  pre-heating  end  there  are  several  openings  at  inter- 
vals into  the  stack  ducts,  and  by  dampering  the  gases  may 
be  drawn  off  before  reaching  the  end  of  the  kiln,  or  they  may 


318 


BURNING  CLAY  WARES. 


be  carried  to  the  end,  thus  shortening  or  extending  the  tem- 
perature conditions  in  the  pre-heating  end  of  the  kiln. 

Another  kiln  has  combustion  chambers  or  tubes  on  each 
side  of  the  ware,  in  principle  equivalent  to  the  Dressier; 
namely,  that  the  combustion  takes  place  in  a tube,  and  the 
ware  is  burned  by  convection  of  the  heat  conducted  through 
the  tube  walls.  Instead  of  using  the  double-walled  combustion 
and  circulating  construction,  extending  it  to  the  end  of  tile 
kiln  by  a simple  duct  and  tubes,  as  in  the  Dressier,  the  com- 
bustion gases  are  taken  through  economizers  on  top  of  the 
kiln,  and  the  air  for  combustion  is  drawn  from  these  econo- 
mizers and  delivered  to  the  burners  and  into  the  combustion 
tubes.  Hot  air  is  also  drawn  from  the  economizers  and  deliv- 
ered into  the  tunnel  under  the  combustion  tubes,  circulating 
around  them,  then  over  and  down  among  the  ware,  and  finally 
it  is  drawn  off  through  the  car  floor  and  out  of  the  kiln,  and 
may  be  returned  to  the  economizer,  thus  making  a complete 
circuit.  The  claim  is  made  that  the  positive  circulation  around 
the  combustion  tubes  and  among  the  ware  is  essential  to  uni- 
form results,  but  the  equipment  is  complicated,  and  if  results 
can  be  obtained  with  a simpler  arrangement,  as  they  seem  to 
be,  the  advantages  will  be  decidedly  in  favor  of  the  simple 
construction. 

The  Harrop  kiln  illustrated  in  Fig.  195  (plan),  Fig.  196 
(cross  section),  Fig.  197  (side  view  showing  furnaces),  and 
Fig.  198  (temperature  curve),  has  several  new  features  in 
car  tunnel  kiln  construction  and  operation. 

The  kiln  is  a simple  direct-fired  tunnel  type  with  staggered 
furnaces  near  the  center  and  the  fuel  may  be  coal,  oil  or 
gas. 

There  are  no  flash  walls  in  front  of  the  furnaces  but 
instead  the  aim  of  the  construction  is  to  direct  the  com- 
bustion gases  from  the  furnaces  among  the  ware  by  deflect- 
ing walls  which  also  serve  as  radiating  surfaces. 

The  circulation  of  the  gases  among  the  ware  in  the  heating 
up  zone  is  simply  and  effectively  attained  by  alternate  off-sets 
in  the  side  walls,  by  battering  these  walls  vertically  and 
by  drop  arches  in  the  kiln  crown.  Gases  will  flow  into  free 
areas  and  hot  gases  rise  to  the  crown  space.  The  alternating 
free  areas  in  consequence  of  the  recessed  side  walls  cause 
the  gases  to  follow  a sinuous  horizontal  course  in  their 
passage  to  the  draft  ports,  passing,  in  part  at  least,  from 
one  enlarged  space  in  the  kiln  wall  across  the  tunnel  to 


BURNING  CLAY  WARES 


319 


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320 


BURNING  CLAY  WARES. 


the  next  following  open  area  in  the  opposite  wall  and  repeat- 
ing to  the  end  of  the  staggered  wall  construction. 

As  the  gases  rise  into  the  crown  space  the  battered  wall 
construction  increases  the  resistance  and  thus  retards  the 
upward  movement;  in  other  words,  the  gases  are  squeezed 
downward  and  inward  among  the  ware,  and  any  gases  col- 
lecing  in  the  crown  space  are  deflected  downward  by  the 
drop  arches. 

The  cars  are  sand-sealed  in  the  usual  manner  and  the 
air  under  the  cars  is  introduced  in  jets  by  fan  pressure. 

A balanced  air  pressure  is  maintained  in  the  tunnel  proper 
and  the  under  car  space.  This  is  accomplished  by  means  of 


dams  in  the  under  car  space  which  increase  the  resistance 
to  the  flow  of  the  air  and  this  resistance  is  measurably 
equivalent  to  the  resistance  of  the  passage  of  the  air  in  the 
upper  tunnel  and  thus  a balance  is  maintained  throughout 
the  length  of  the  kiln. 

Air  for  cooling  the  ware  and  for  secondary  combustion  is 
introduced  at  the  discharge  end  of  the  kiln  by  the  same 
fan  which  supplies  the  under  tunnel  air.  It  has  been  found 
that  the  hot  air  in  the  cooling  end  of  the  tunnel  tends  to 
drift  back  to  the  discharge  end  in  the  upper  part  of  the 
kiln,  while  the  in-going  air  is  an  under  current  in  the  lower 
level  of  the  tunnel.  This  unsatisfactory  air  movement  is 


BURNING  CLAY  WARES. 


321 


counteracted  by  introducing  the  cold  air  horizontally  under 
the  crown  by  means  of  a metal  inner  crown  sheet  annular 
with  the  crown  and  several  inches  below  it.  The  air  fan 
has  several  functions:  To  introduce  air  into  the  tunnel  for 
cooling  and  secondary  combustion;  to  put  the  cooling  air 
under  the  cars;  to  force  air  through  a duct  on  top  of  the 
kiln  for  primary  combustion  use  in  the  furnaces  or  to  be 
admitted  to  any  part  of  the  kiln  for  any  purpose. 

The  draft  fan  is  at  the  charging  end  of  the  kiln  and 
connects  with  the  kiln  through  an  under  cross  duct  with 
vertical  ducts  to  horizontal  ducts  in  the  side  walls  at  the 
car  deck  level. 

There  are  a series  of  ports  in  these  horizontal  ducts  which 
enable  the  operator  to  remove  the  gases  practically  at  the 
discharge  end  or  at  any  point  in  the  length  of  the  ducts. 


Figure  197 

This  is  a provision  for  adjustment  in  the  kiln  operation  to 
adapt  it  to  the  need  of  the  ware  in  the  early  stages  of  the 
water-smoking  period,  and  once  adjusted  no  further  changes 
are  likely  to  be  required. 

The  draft  conditions  in  the  kiln  are  controlled  at  the  fan. 

The  heat  curve,  Fig.  198,  shows  practically  a straight 
line  increase  in  temperature  through  the  heating  up,  or 
oxidation  zone  to  the  furnace  zone,  then  it  is  held  with 
slight  variation  through  the  furnace  zone,  dropping  rapidly 
through  the  higher  temperature  of  the  cooling,  and  then 
slowly  through  the  annealing  section.  This  curve  is  taken 
from  a kiln  burning  porcelain  in  which  the  problems  of  water- 
smoking and  oxidation  are  far  different  from  those  in  many 
common  wares.  Kilns  to  burn  such  wares  must  make  better 
provision  for  oxidation  in  order  that  this  part  of  the  burning 
may  keep  pace  with  the  final  burning. 


322 


BURNING  CLAY  WARES. 


Professor  Harrop  meets  this  requirement  by  introducing 
a preliminary  set  of  furnaces  near  the  point  where  oxidation 
begins,  and  provides  for  the  introduction  of  pre-heated  air 
between  the  main  furnace  zone  and  the  preliminary  furnaces. 
The  length  of  the  kiln  is  increased  to  the  extent  of  this 
oxidation  section.  The  heat  curve  will  have  the  gradual 
rise  through  the  water-smoking  zone  as  shown  in  Fig.  198, 
up  to  the  oxidation  zone,  then  a slow  rise  through  the  oxida- 
tion zone  into  the  main  furnace  zone,  followed  by  the  quick 
cooling  to  the  annealing  temperature,  then  slow  cooling  to 
the  end.  Clayworkers  recognize  the  fact  that  in  the  majority 
of  wares,  the  cooling  from  high  temperatures  to  a low  red 
heat  may  be  done  rapidly  without  damage  to  the  ware,  and 
this  rapid  cooling  beyond  the  furnace  zone  as  shown  in  the 
heat  curve  is  accomplished  by  the  introduction  of  air  in  this 
section  of  the  kiln  through  flues  provided  for  this  purpose, 
but  if  the  ware  will  not  stand  such  treatment  the  air  may 
be  cut  out. 

There  is  an  interesting  car  tunnel  kiln  in  successful  opera- 
tion on  small  pieces  of  electrical  porcelain.  The  ware  is  placed 
on  fire  clay  trays  on  the  cars.  The  kiln  is  an  open  fire  type, 
but  instead  of  a direct  flame  contact  the  heat  is  deflected  to 
the  ware,  which  is  exposed  to  the  combustion  gases.  It  may 
be  noted  that  two  of  the  kilns  previously  mentioned  make 
use  of  deflected  heat. 

The  car  tunnel  kiln  is  developing  along  three  lines : 

(1)  The  open  fire  kiln,  with  the  combustion  gases  in 
direct  contact  with  the  ware,  and  the  latter  may  be  in  open 
setting  or  enclosed  in  saggers. 

(2)  The  open  fire  kiln  in  which  deflecting  walls  are  used 
to  direct  the  combustion  gases  to  the  ware  set  either  exposed 
or  in  saggers. 

(3)  The  muffled  kiln,  in  which  the  ware  may  be  set  in  sag- 
gers, but  only  as  a matter  of  convenience  in  handling  it. 

The  first  two  types  are  very  closely  akin.  The  same  kiln 
may  have  a perforated  flash  wall  between  the  furnace  and  the 
ware  and  thus  operate  on  the  principle  of  the  first  type,  or 
the  perforated  wall  may  be  removed  and  deflecting  walls  built 
in  such  a way  as  to  deflect  the  gases  from  their  natural  course 
down  on  or  among  the  ware. 


BURNING  CLAY  WARES. 


323 


CHAPTER  XII. 

BURNING  A DOWN  DRAFT  KILN. 

DOWN  draft  kiln  operation  has  four  distinct  stages, — 


water-smoking,  oxidation,  heating  up,  and  driving  the 


heat  to  the  bottom,  or  heat  soaking.  Fire  flashing  is 


a fifth  stage  in  the  burning. 

The  ware  from  the  dryer  is  never  fully  dry.  There 
remains  a fraction  of  a per  cent,  up  to  three  or  more  per 
cent,  of  moisture  and  two  to  three  per  cent,  of  free  water 
in  such  form  that  it  cannot  be  removed  by  dryer  tempera- 
tures. 

Wares  are  often  set  before  they  are  apparently  dry,  but 
even  though  they  are  seemingly  dry,  they  contain  water  ap- 
proximating five  per  cent,  which  must  be  removed  in  the 
kiln  operation.  The  removal  of  this  water  content  is  the 
water-smoking  stage  in  the  burning,  and  the  rate  of  its  re- 
moval depends  upon  the  behavior  of  the  ware  under  such 
drying  conditions  and  upon  the  efficiency  of  the  kiln  con- 
struction. 

Slabbing  of  sewer  pipe  and  large  drain  tile  starts  in  the 
water-smoking;  steaming  with  its  resultant  more  or  less 
rotten  ware  and  subsequent  discoloration,  is  a water-smoking 
trouble. 

With  some  wares  the  fires  may  be  advanced  as  rapidly 
as  it  is  possible  to  build  them  up,  and  in  other  wares,  espe- 
cially dry  pressed  products,  the  kiln  temperatures  must  be 
held  very  low  for  a period  of  five  or  more  days,  and  in  some 
instances  up  to  thirty  days,  to  safely  remove  the  moisture. 
The  rate  of  water-smoking  must  be  determined  for  each  ware, 
and  the  fires  manipulated  to  attain  the  desired  results.  The 
usual  custom  is  to  build  a small  fire  in  the  ash  pit  of  each 
furnace,  preferably  of  wood,  coke,  smokeless  coal,  or  gas, 
but  soft  coals  are  often  used.  The  objection  to  soft  coal 
is  the  large  volume  of  smoke  which  soots  the  damp  ware 
and  often  the  draft  throughout  the  burn  is  choked  by  the 
sooted  draft  spaces.  The  furnace  doors  or  mouths  should  be 
left  open  and  as  much  draft  maintained  as  possible.  Un- 
fortunately where  the  draft  is  by  a stack  we  have  very  weak 
draft  during  the  water-smoking  stage  in  consequence  of  the 
stack  being  cold. 


324 


BURNING  CLAY  WARES. 


A kiln  with  a single  stack  is  objectional,  and  in  building 
a battery  of  kilns  it  is  better  to  have  one  stack  for  two, 
three,  or  four  kilns,  but  with  an  independent  flue  for  each 
kiln.  In  this  way  we  get  the  benefit  of  conducted  heat  from 
the  combustion  gases  of  the  burning  kilns,  but  in  any  event 
such  a stack  does  not  cool  to  the  extent  of  a single  stack, 
and  we  get  the  advantage  of  this  heat  in  starting  the  water- 
smoking. If  the  situation  is  such  that  single  stacks  are 
unavoidable,  they  should  be  built  with  small  furnaces  at  the 
base  to  heat  up  the  stack  and  thus  give  us  a good  draft 
through  the  kiln  in  the  early  stages  of  the  water-smoking. 
A better  plan  would  be  to  have  a fan  connection  for  the  start 
of  the  kiln  burning. 

Fig.  199  shows  the  heat  curves  for  two  kilns,  both  burn- 
ing bricks,  and  as  it  happens  both  clays  stand  a lot  of  abuse 
in  water-smoking  and  oxidation.  These  curves  are  shown 
in  the  solid  line  and  the  dotted  line.  Practically  complete 
removal  of  the  moisture  is  attained  at  a temperature  around 
400  deg.  Fah.  to  450  deg.  In  the  kiln  shown  by  the  solid 
line  the  water-smoking  was  finished  in  16  hours  and  the 
dotted  line  kiln  was  ready  for  oxidation  in  24  hours. 

If  one  has  a pyrometer — and  every  kiln  should  be  equipped 
with  such  an  instrument — and  the  burner  has  determined  the 
safe  rate  of  advance  of  the  heat  in  the  top  of  the  kiln,  he  can 
safely  control  the  fires  to  get  satisfactory  results,  but  if  he 
has  no  such  guide,  he  must  feel  his  way  by  observation  of 
the  volume  of  steam  coming  from  the  top  of  the  stack  and 
by  means  of  an  iron  rod  thrust  into  the  kiln  at  different 
levels,  noting  the  condensation  on  the  rod.  If  the  moisture 
driven  from  the  ware  is  not  removed  from  the  kiln,  but 
instead  condenses  and  soaks  into  the  ware  in  the  bottom  of 
the  ki  n,  the  result  will  be  a lot  of  damaged  ware  in  the 
bottom,  and  we  have  seen  many  burns  in  which  it  was 
evident  the  water-smoking  had  been  advanced  too  rapidly  in 
that  the  kiln  floor,  kiln  flues,  and  stack,  did  not  amply 
provide  for  the  removal  of  the  steam.  Where  such  con- 
ditions exist  the  water-smoking  must  be  correspondingly 
slower  to  prevent  steaming  the  bottom  ware. 

The  dash  and  dot  curve  in  Fig.  199  illustrates  slower 
water-smoking  in  which  we  have  indicated  68  hours  for  this 
stage  in-  the  process  and  for  some  wares  this  would  be 
rapid  work. 

In  the  other  extreme,  one  operation — solid  bricks  set  high 
and  close — the  burning  was  completed  in  72  hours,  but  few 
clays  will  stand  such  rapid  work.  The  kiln  in  this  instance 
had  the  advantage  of  fan  draft  in  the  water-smoking  and 
throughout  the  burn,  and  the  clay  stood  a lot  of  abuse  in 
water-smoking,  besides  requiring  no  oxidation,  and  also  it 
had  such  a long  burning  range  that  it  was  not  necessary  to 
follow  the  usual  procedure  in  working  the  heat  to  the  bottom 
but  instead  the  fires  were  kept  under  full  head  until  the 
bricks  in  the  bottom  of  the  kiln  were  hard. 


BURNING  CLAY  WARES. 


325 


The  usual  burning  periods  are  three  to  five  days  for 
hollow  ware,  five  to  nine  days  for  stiff  mud  bricks,  and  eight 
to  twelve  days  for  dry  pressed  bricks. 

In  general  practice  the  fires  are  built  up  to  full  fire  by 
the  time  the  water-smoke  is  off,  and  with  full  fire  begins  the 
oxidation.  The  oxidation  consists  in  burning  out  the  carbon 
in  the  clay  and  roasting  out  the  sulphur,  otherwise  the 
product  will  be  black  cored  and  in  the  finishing  stages  the 
product  will  be  bloated.  The  kiln  is  under  full  fire  and  with 
all  the  draft  it  can  get,  but  the  fires  are  kept  open  to  take 
in  as  much  excess  air  as  possible.  If  the  furnaces  are  of 


Figure  199. 


the  grateless  type  we  do  not  close  the  firing  mouths  with  coal 
as  in  the  later  stages  of  the  burning,  or  if  the  furnaces  are 
provided  with  doors,  the  latter  are  left  partly  open.  The 
firing  should  be  light  and  frequent,  keeping  in  mind  that  we 
wish  to  advance  the  heat  slowly  and  to  admit  as  much 
excess  air  as  may  be  taken  in.  It  is  the  hot  air  that  burns 
out  the  carbon  and  sulphur  in  the  clay.  We  could  carry 
the  heat  up  to  800  degrees,  more  or  less,  and  hold  it  until 


326 


BURNING  CLAY  WARES. 


oxidation  was  complete,  but  it  is  better  to  keep  the  heat 
advanced  slowly  to  the  end  of  the  oxidation  and  then  jump 
as  quickly  as  possible  to  the  final  stage  of  the  burning. 

The  “blue  smoke”  from  the  stack,  following,  the  water- 
smoke,  is  the  visual  evidence  of  the  oxidation,  and  the  end 
is  determined  by  trial  pieces  taken  from  the  kiln  and  noting 
whether  the  black  core  has  fully  disappeared.  In  one  opera- 
tion we  noticed  that  after  the  black  core  had  disappeared, 
there  remained  a saffron  colored  core  which  had  not  pre- 
viously been  considered.  Extending  the  oxidation  period  24 
hours  until  the  yellow  core  was  dissipated,  resulted  in  an 
improvement  of  five  or  more  points  in  the  rattler  test  in  this 
paving  brick  product. 

It  is  therefore  not  always  safe  to  assume  that  oxidation 
Is  complete  with  the  disappearance  of  the  black  core. 

Oxidation  takes  place  rapidly  at  a low  red  heat  and  in 
general  the  heat  should  not  be  allowed  to  get  much  above 
1200  degrees  until  after  the  black  core  is  fully  out.  Carbon 
particularly  should  be  driven  out  at  low  temperatures  be- 
cause at  higher  temperatures  there  is  a greater  tendency 
to  cracking  of  the  gases  and  the  deposition  of  carbon  instead 
of  its  removal,  and  at  high  temperatures  regardless  of  the 
c uantity  of  excess  air,  carbon  in  the  clay  cannot  be  burned 
out. 

Sulphur  may  be  removed  at  higher  temperatures  but  it 
must  be  out  before  vitrification  begins.  In-  one  factory  using 
a shale  high  in  carbon  and  sulphur  it  was  the  practice  to 
hold  the  temperature  around  1200  degrees  until  the  carbon 
was  out  and  with  it  a lot  of  the  sulphur,  and  this  was  in- 
dicated by  the  color  of  the  smoke  from  the  stack. 

With  the  disappearance  of  the  blue  smoke,  the  tempera- 
ture was  advanced  to  1,400  degrees,  which  resulted  in  a large 
volume  of  blue  smoke,  and  this  temperature  was  held  until 
the  smoke  was  off,  when  the  temperatures  were  further  ad- 
vanced to  1600  degrees  and  again  held  till  the  blue  smoke 
was  off. 

The  combined  water  in  the  clay  begins  to  come  off  rapidly 
around  800  degrees  but  it  does  not  require  any  change  in  the 
burning  operation.  Following  the  removal  of  the  combined 
water,  the  ware  begins  to  shrink,  if  it  has  any  shrinkage, 
and  the  progress  of  the  burn  in  many  factories,  especially 
those  not  equipped  with  pyrometers,  is  determined  by  the 
rate  of  the  settle  of  the  mass  of  ware.  The  guides  in  burn- 
ing, besides  the  pyrometer,  are  the  water-smoke,  the  blue 
smoke,  the  black  and  yellow  cores,  the  settle,  and  trial 
pieces. 

When  oxidation  is  complete  the  fires  should  be  closed  at 
the  furnace  mouths  and  the  heat  may  be  advanced  to  the 
finishing  stage  as  rapidly  as  possible. 

This  is  the  final  heating  up  and  it  is  the  heavy  firing 
period  during  the  burning.  The  fires  should  be  kept  clean 
and  in  the  best  possible  condition  to  get  a maximum  value 
out  of  the  fuel 


BURNING  CLAY  WARES. 


327 


When  the  finishing  temperature  is  attained,  or  nearly 
so,  on  top  we  begin  the  process  of  working  the  heat  to  the 
bottom.  The  dampers  are  lowered  to  check  the  draft  and 
thus  we  increase  the  pressure  in  the  kiln,  or  more  properly 
equalize  the  pressure  and  under  this  equal  pressure  the  heat 
drifts  to  all  parts  of  the  kiln.  The  heat  should  be  held  at 
the  finishing  temperature,  or  practically  so,  throughout  this 
final  period,  and  this  can  be  done  by  firing  lighter,  and 
oftener,  if  necessary.  The  furnace  mouths,  if  the  furnaces 
are  the  grateless  or  inclined  grate  type,  should  be  lightly 
closed  with  coal,  permitting  the  bed  to  burn  down  quickly  to 
a small  opening  over  the  fuel  bed. 

A single  shovelful,  or  two  at  most,  suffices  for  each  fire 
and  it  is  seldom  necessary  to  lessen  the  intervals  of  firing, 
in  fact  we  may  often  increase  the  interval. 

If  it  is  a new  operation  one  must  work  slowly  through  all 
the  stages  of  the  burning.  It  is  better  to  take  ten  or  more 
days  in  the  early  burns  than  to  discourage  the  investors  in 
the  factory  with  bad  burns.  The  curve  to  be  followed  is 

along  the  line  of  the  dash  and  dot  curve  shown  in  Fig.  199, 

and  one  may  go  even  slower  than  this  depending  upon  the 
exhibits  of  the  burn  previously  mentioned. 

If  the  first  burn  is  successful  throughout,  one  may  begin 
to  shorten  the  periods,  one  at  a time.  A day  more  or  less 
may  be  clipped  from  the  water-smoking  and  repeated  until 
the  bottom  ware  shows  some  effect  of  steaming.  This  is 
beyond  the  limit  and  the  period  must  be  increased  until  no 
steaming  is  apparent.  The  next  step  is  to  shorten  the 

oxidation  period  in  the  same  manner  until  some  bloated  ware 
shows  that  the  oxidation  has  been  too  fast.  The 

dash  and  dot  curve  shows  relatively  slow  oxidation.  The 
heating  up  following  the  oxidation  is  usually  simply  a matter 
of  getting  the  most  out  of  the  fuel,  and  the  final  tempera- 
ture requires  careful  handling  until  satisfactory  trials  are 
obtained  from  the  bottom  of  the  kiln. 

Fire  flashing  usually  starts  toward  the  end  of  the  finish- 
ing heat  and  merely  consists  in  working  with  dampers  lowered 
in  a greater  degree  and  keeping  the  fires  fully  closed  all  the 
time.  It  is  often  designated  as  the  “Smoking”  stage  because 
during  it  the  volume  of  smoke  is  greatly  increased,  but  it  is 
more  properly  a reduction  stage  and  smoke  is  not  essential 
but  unavoidable.  The  great  danger  is  that  we  may  advance 
the  temperature  beyond  that  which  the  ware  will  safely 
stand. 

In  consequence  of  this  danger,  we  must,  with  many  clays, 
work  with  an  excess  reducing  condition,  first  to  get  the 
reducing  condition  which  produces  the  flash,  and  second  to 
get  it  in  such  degree  that  the  flame  temperature  and  thereby 
the  temperature  in  the  kiln  is  not  increased. 

In  the  water-smoking  and  oxidation  stages  we  are  work- 
ing with  low  temperatures  and  large  air  excess;  in  the  heat- 
ing we  endeaver  to  get  as  nearly  perfect  combustion  as  pos- 
sible to  get  a maximum  heat  value  out  of  the  fuel;  in  the 


328 


BURNING  CLAY  WARES. 


soaking  period  we  work  with  high  temperatures,  slow  draft, 
and  open  fires  and  the  excess  air  keeps  the  temperature 
from  running  too  high;  in  the  flashing  we  used  closed  fires, 
slow  draft  and  insufficient  air  which  prevents  excessive 
temperature.  If,  in  the  flashing,  we  changed  the  soaking  pe- 
riod firing  back  to  the  heating  up  firing  except  retaining  the 
slow  draft,  we  would  rapidly  advance  the  temperature  which 
few  wares  would  stand.  Instead  we  go  beyond  the  heating  up 
firing  and  so  damper  the  furnaces  with  coal  that  the  com- 
bustion is  far  below  perfect  conditions  and  the  flame  tem- 
peratures are  lower  and  thus  we  can  control  the  kiln  tem- 
peratures, but  in  doing  so  we  get  a lot  of  smoke  we  do  not 
need. 

The  flashing  period  varies  from  12  to  24  hours,  depending 
upon  the  clay  and  the  degree  of  flash  desired. 

COLORATION,  DISCOLORATION,  AND  OTHER  BURNING 

There  are  many  burning  effects,  intentional  and  otherwise, 
concerning  which  frequent  inquiries  are  received,  and  a dis- 
cussion of  a few  of  the  more  common  ones  may  be  pertinent. 

Scum. 

The  almost  universal  bug-bear  of  the  clay-worker  is  scum. 
It  comes  up  again  and  again  as  a kiln  trouble  because  it  be- 
comes apparent  in  the  kiln,  and  we  have  made  many  in- 
vestigations of  it.  It  is  chiefly  a dryer  trouble  which  has 
been  discussed  in  “Scumming  and  Efflorescence”  by  the 
author  and  there  are  several  available  published  articles 
relative  to  it. 

The  ordinary  scum  is  a dirty  white  coating  which  comes 
to  the  surface  of  the  ware  in  the  drying  and  becomes  per- 
manent in  the  burning.  Where  such  trouble  occurs  the  first 
step  should  be  to  determine  whether  it  originates  in  the 
dryer  or  in  the  kiln.  If  the  product  is  brick,  a brief  study 
of  the  scummed  cross  bars  on  the  faces  and  backs  of  the 
bricks,  corresponding  to  the  setting,  will  determine  the  origin 
of  the  trouble.  If  these  marks  correspond  with  the  setting 
on  the  dryer  cars  and  not  with  the  kiln  setting,  the  trouble 
goes  back  to  the  dryer  with  which  we  are  not  concerned 
in  the  burning. 

The  examination  of  the  burned  product  in  the  kiln  fre- 
quently reveals  that  the  coating  occurs  in  both  dryer  and  kiln, 
but  occasionally  it  is  clearly  a kiln  trouble. 

In  the  majority  of  instances  the  scumming  minerals  are 
in  the  clay  or  shale  and  the  water  used  in  pugging. 

The  common  scum  is  sulphate  of  lime  (gypsum).  Opera- 
tors have  often  observed  glassy  plates  in  the  clay  or  shale 
which  to  them  “looked  like  isinglass”  but  instead  these 
crystal  plates  are  gypsum,  and  the  proof  is  that  they  can 
be  scratched  by  one’s  finger  nail.  Ground  waters  almost 
invariably  carry  some  sulphate  of  lime  which  produces  a 
tough  adhesive  scale  in  steam  boilers. 


BURNING  CLAY  WARES. 


329 


If  a careful  investigation  were  made  it  is  doubtful  if  there 
would  be  found  one  shale  out  of  every  hundred  that  is  free 
from  gypsum.  If  the  scum  minerals  are  so  universal  it  may 
be  asked  why  it  is  that  many  clays  and  shales  in  use  do  not 
scum,  or  at  most  only  occasionally. 

The  explanation  is  that  the  clay  mass  absorbs  more 
or  less  of  the  salts  of  whatever  kind  and  in  drying,  these 
absorbed  salts  do  not  come  to  the  surface.  The  surface  scum 
is  the  excess  salts  in  the  clay  mass. 

The  elements  of  scum  are  therefore  usually  found  in  the 
clay  or  water,  or  both.  It  may  be  present  in  neither  in 
sufficient  quantity  to  cause  surface  scum,  but  the  base  element 
may  be  in  the  clay  and  the  acid  element  in  the  water,  or 
vice  versa,  and  when  the  two  are  brought  together  an  excess 
of  scum  mineral  may  be  developed. 

Scum  often  develops  in  the  dryer  when  combustion  gases 
either  direct  or  by  leakage,  get  into  the  dryer.  The  shale 
or  clay  may  contain  lime  insoluble  in  water — lime  carbon- 
ate, for  instance — and  this  is  converted  into  sulphate  by 
sulphur  gases  in  a humid  atmosphere  in  contact  with  the 
drying  ware.  If  the  ware  is  not  scummed  in  the  dryer,  but 
does  scum  in  the  kiln,  the  investigator  should  determine 
whether  the  effect  is  not  due  to  setting  wet  ware.  If  the 
kiln  fuel  is  coal,  we  get  sulphur  gases  from  it  and  if  the 
ware  is  set  wet,  we  get  the  dryer  conditions  above  mentioned, 
and  the  correction  is  evident. 

It  may  be  asked  how  sulphur  gas  and  water  vapor,  es- 
sentially a gas,  in  the  kiln  among  the  ware  but  not  within  the 
ware,  can  penetrate  the  ware,  disintegrate  the  lime  minerals 
and  dissolve  the  lime  and  subsequently  come  to  the  surface 
in  a liquid  form,  which  it  must  do  to  bring  the  dissolved  salts 
to  the  surface?  Sulphuric  acid  forms  when  sulphur  dioxide 
and  water  vapor  are  mixed  or  when  the  dioxide  is  brought 
in  contact  with  a damp  surface.  It  volatilizes  at  a much 
higher  temperature  than  water,  and  although  water  vapor 
may  be  leaving  the  ware,  any  sulphuric  acid  developed  is 
being  absorbed  by  the  ware  and  it  with  any  dissolved  salts 
are  brought  to  the  surface  later  in  the  water-smoking  period. 

In  this  connection,  but  aside  from  the  subject  under  dis- 
cussion, we  may  mention  a sulphur  gas  trouble  which  was 
experienced  by  a number  of  the  early  users  of  continuous 
kilns,  before  advanced  heating  flues  were  adopted. 

The  ware  when  burned  to  vitrification,  or  approaching 
vitrification,  was  swelled, — not  bloated  as  we  understand 
bloating, — yet  perfect  in  shape  but  a larger  size  than  the  same 
ware  burned  in  periodic  kilns.  The  soft  burned  ware  was 
normal  in  size  but  the  hard  burned  product  often  was  larger 
than  the  dry  ware  although  the  normal  burning  shrinkage 
should  have  been  six  or  more  per  cent. 

In  such  kilns  we  had  not  only  the  sulphur  from  the  fuel 
but  often  a far  greater  volume  of  sulphur  gas  from  the  shale 
or  clay  and  these  gases  were  going  forward  into  low  tempera- 


330 


BURNING  CLAY  WARES. 


ture  water-smoking  compartments.  The  conditions  for  the 
development  of  sulphuric  acid  were  ideal,  and  the  effect 
on  the  product  was  as  noted  above. 

No  satisfactory  explanation  of  this  has  ever  been  made, 
so  far  as  we  know.  Undoubtedly  mineral  sulphates  were 
developed  in  the  ware  which  had  little  or  no  effect  upon  the 
soft  burned  ware,  and  it  likely  that  at  vitrifying  tempera- 
tures, the  sulphates  were  dissociated,  and  likely  we  got  a 
result  similar  to  black  coring  and  bloating  with  which  all 
clayworkers  are  familiar,  yet  apparently  there  was  no  visible 
vesicular  structure  such  as  we  get  in  bloated  ware.  In- 
cidentally it  may  be  mentioned  that  clays  containing  lime 
pebbles  are  often  burned  in  continuous  kilns  and  the  burned 
product  does  not  “pop,”  as  it  does  when  burned  in  periodic 
kilns. 

Returning  to  the  subject  of  scumming,  it  is  evident  that 
the  ordinary  scum  develops  in  the  dryer,  and  if  the  ware  is 
properly  dried  and  not  scummed,  it  will  not  scum  in  the 
kiln. 

Two  rare  discolorations  have  come  to  our  attention, — one 
of  them  in  several  operations  and  the  other  in  a single  in- 
stance. The  former  is  an  evanescent  white  coating  which 
appears  on  the  surface  of  the  bricks, — even  vitrified  bricks, — 
after  they  are  burned.  When  the  kiln  is  first  opened  and 
still  hot,  the  ware  is  clean  but  as  cooling  proceeds  the  white 
coating  appears.  In  one  instance  the  bricks  were  clean  when 
taken  from  the  kiln  but  became  heavily  coated  shortly  after, 
without  moisture  except  such  dampness  as  they  might  gather 
from  the  atmosphere  during  a summer  day.  An  analysis 
of  this  coating  showed  it  to  be  an  alkaline  sulphate, — an  alum 
if  you  please,  but  it  is  not  fully  clear  to  us  how  this  coating 
develops. 

In  three  instances  we  found  that  the  trouble  appeared 
when  the  bricks  has  been  burned  with  wet  dirty  coal  from 
an  old  mine.  In  two  of  these , instances  we  suggested  that 
clean,  freshly  mined  coal  be  tried  and  the  result  was  that 
the  trouble  was  overcome.  One  operator  after  getting  clean 
ware  from  good  coal,  went  back  to  the  cheaper  coal  which  he 
got  from  a near  by  old  mine,  and  the  efflorescence  reap- 
peared. In  one  operation  the  product  was  a vitrified  flashed 
brick  and  it  is  inconceivable  that  the  efflorescence  could 
have  come  from  within  the  product,  especially  in  view  of  the 
fact  that  there  is  no  moisture  except  atmospheric  vapor. 

Evidently  the  alkaline  sulphate  is  a surface  coating  and 
it  seems  to  us  that  it  must  come  from  the  coal  ash. 

Its  visibility  and  evanescence  are  due  to  the  action  of  am- 
monia in  the  atmosphere,  the  effect  of  which  is  to  produce  a 
white  effloresence  on  the  alkaline  sulphate. 

There  likely  is  present  in  the  surface  of  the  ware  an  invis- 
ible crystalline  alkaline  sulphate  which  when  acted  upon  by 
ammonia  produces  a white  flocculent  ammonia  alum,  and  this 
is  the  white  coating  which  appears  under  the  conditions  given 


BURNING  CLAY  WARES. 


331 


above.  It  is  very  soluble  and  quickly  disappears  in  the 
presence  of  excess  moisture. 

In  this  connection  we  may  mention  a vitrified  flashed 
brick  product  which  does  not  effloresce  in  the  wall,  but  in  two 
jobs  effloresced  badly,  and  these  two  jobs  were  ice  manu- 
facturing plants.  This  efflorescence,  it  seems  to  us,  is  analo- 
gous to  the  above  described  phenomenon,  because  of  the 
ammonia  vapor  in  the  ice  factories. 

The  white  coating  which  we  have  described  and  which  is 
due  to  a condition  developed  in  the  kiln,  must  not  be  con 
fused  with  the  white,  yellow  and  green  coatings  which  appear 
on  wares  on  the  yard  and  often  after  the  product  is  laid 
in  the  wall.  These  are  quite  another  story  which  goes  back 
to  the  clays  when  the  coatings  are  developed  from  the  pro- 
duct and  not  from  extraneous  sources,  nor  are  these  efflores- 
cences related  to  scumming,  at  least  not  closely  related. 

The  other  unusual  coating  mentioned  previously  had  all 
the  appearance  of  common  ordinary  scum.  An  examination  of 
the  bricks  in  the  kiln  showed  masses  of  the  ware  badly 
scummed  and  the  scum  flared  out  on  the  sides  of  the  bricks 
where  the  gases  came  up  through  the  checker  work  and  the 
white  cross  bars  on  the  backs  of  the  bricks  corresponded 
to  the  kiln  setting. 

It  was  clearly  a case  of  kiln  scumming.  The  product  from 
the  dryer  was  clean  in  spite  of  the  fact  that  the  drying  was 
done  with  combustion  gases  from  the  boilers  and  gas  from 
the  producers  from  which  gas  was  also  drawn  to  burn  the 
kilns.  This  was  the  puzzling  problem.  If  the  combustion 
gases  would  not  scum  the  product  in  drying  why  should  they 
develop  scumming  in  the  kiln  work?  A mineralogical  exami- 
nation by  an  authority  in  this  work  revealed  that  the  coating 
was  almost  entirely  silica, — just  white  sand.  If  this  is  correct 
the  explanation  is  simple.  The  bricks  were  closely  set  in 
an  updraft  kiln  and  platted  with  a tight  flat  course  instead 
of  the  usual  spaced  course  covered  by  a tight  course,  and  the 
openings  were  small  and  far  apart  instead  of  the  customary 
opening  of  each  alternate  brick  in  the  tight  course.  The 
result  was  that  the  bricks,  especially  in  the  patches  where 
the  draft  was  sluggish,  were  steamed  in  conjunction  with 
sulphur  gases. 

Any  chemist  is  familiar  with  the  rational  analysis  of  clays 
by  which  practically  all  the  minerals  in  the  clay  except 
free  silica  are  dissolved  by  hot  sulphuric  acid.  This,  it 
seems  to  us,  is  what  happened  in  the  above  described  kiln 
operation.  The  surface  minerals  except  sand  were  dissolved 
and  carried  into  the  bricks  and  the  final  evaporation  took 
place  below  the  surface,  or  at  least  under  the  sand,  leaving 
the  latter  as  a dirty  white  coating. 

Manufacturers  of  clay  wares  are  familiar  with  a number 
of  discolorations  in  consequence  of  steaming  in  a sulphur 
acid  atmosphere. 

In  the  manufacture  of  gray  bricks  using  manganese  for 
the  gray  color,  light  edges  bordered  by  a streak  darker  than 


332 


BURNING  CLAY  WARES. 


the  normal  color  of  the  brick,  were  frequently,  and  on  some 
yards  invariably  produced,  when  water-smoking  with  coal 
or  coke. 

We  have  had  similar  experience  with  light  red  bricks 
especially  when  burning  the  product  in  continuous  kilns  with- 
out advanced  heating  flues  and  occasionally  in  down  draft 
kilns  when  the  draft  was  sluggish,  or  the  kiln  bottom  became 
choked  by  soot  or  crushing  of  the  bottom  courses. 

We  have  often  seen  buff  bricks  badly  streaked  and  coated 
with  red, — the  sides  and  cross  bars  of  the  setting  and  any 
part  of  the  product  from  which  the  final  evaporation  took 
place. 

Fire-proofing  from  buff  burning  clays  when  set  on  solid 
kiln  bottoms  often  develop  a reddish  color  in  the  bottom 
tiles. 

All  of  these  effects  are  produced  by  sulphur  gas  and 
moisture.  The  reddish  colors  in  the  buff  burning  product  is 
due  to  the  disintegration  and  solution  of  the  iron  minerals  by 
the  sulphuric  acid  and  the  iron  stain  is  left  as  a red 
oxide  coating  on  the  surface  of  the  ware  or  within  the  ware 
as  the  final  evaporation  of  the  acid  moisture  takes  place  from 
the  surface  or  below  the  surface.  The  streaked  edges  in  the 
gray  bricks  were  due  to  the  solution  of  the  manganese  which 
was  taken  into  the  bricks  and  finally  left  as  a streak  at  a 
depth  below  the  surface  where  the  moisture  was  converted 
into  vapor. 

Many  discolorations  are  due  to  sulphur  gases  and  conden- 
sation on  the  ware  in  the  colder  parts  of  the  kiln,  and  im- 
provement, if  not  complete  correction  will  come  from  slower 
water-smoking,  better  circulation,  more  open  kiln  bottoms, 
and  stronger  draft. 

Fire  Flashing. 

Fire  flashing  is  a color  effect  produced  by  reducing  kiln 
atmospheres,  but  more  than  this,  the  flashed  face  must  be 
in  contact  with  the  moving  gases,  or  in  other  words,  in  the 
path  of  the  flame. 

A buff  burning  clay  in  a reducing  kiln  atmosphere  but 
not  in  flame  contact  will  develop  a grayish  color  usually 
speckled  with  iron  yet  when  exposed  to  direct  flame  contact 
the  result  is  a russet  colored  flash.  The  grayish  color  in  the 
reducing  atmosphere  is  due  to  the  reduction  of  the  iron  to  the 
ferrous  state  and  its  combination  with  silica  in  which  the 
larger  aggregates  produce  visible  black  spots  of  the  iron  sili- 
cate and  similarly  the  disseminated  and  finely  divided  iron, 
but  the  specks  are  so  minute  that  they  are  invisible  and  yet 
to  them  is  due  the  grayish  color,  and  the  effect  will  fully  per- 
meate the  mass  if  sufficient  time  is  given  for  the  reduction. 

When  the  surface  of  the  ware  is  exposed  to  flame  com 
tact  the  result  is  a russet  colored  flash  and  exclusively  a 
surface  effect. 

Mr.  H.  B.  Henderson  in  a paper  before  the  American 
Ceramic  Society  has  shown  that  the  russet  color  is  due  to 


BURNING  CLAY  WARES. 


333 


amber  colored  hexagonal  crystals  which  owe  their  develop- 
ment to  carbon, — likely  graphitic  carbon, — from  the  kiln 
gases,  and  probably  in  a nascent  state  from  the  cracking  of 
the  gases,  especially  methane. 

The  color  of  salt  glazed  ware  and  unglazed  fire  flashed 
ware  is  due  to  such  crystals  and  analyses  of  several  such 
glazes  show  a small  content  of  carbon. 

It  is  not  believed  that  carbon  enters  largely  into  the  con- 
struction of  the  crystals  nor  that  the  color  is  due  to  carbon, 
but  instead  it  is  the  agent  which  starts  the  crystal  and 
perhaps  continues  to  influence  its  growth  throughout  the 
period  of  development.  The  color  is  thought  to  be  due  to 
iron  either  dissolved  in  the  crystal  or  as  a constitutent  of 
the  crystal.  The  intensity  of  the  color  may  be  due  to  the 
solution  of  the  iron. 

A small  content  of  iron  in  glass  sand  has  no  color  effect 
on  the  sand  but  when  the  sand  is  fused  into  glass  the  color 
effect  of  the  iron  is  very  marked,  and  similarly  the  iron 
which  gives  a buff  color  to  fire  clay  wares  will  be  intensely 
deeper  in  a crystal  glaze,  or  it  may  be  that  the  color  is 
largely  due  to  light  refraction.  With  this  problem  we  are 
not  concerned. 

The  analyses  of  the  glazes  do  not  justify  the  assumption 
that  carbon  is  a major  constituent  of  the  crystals,  and  the 
only  explanation  is  that  it  is  the  agent  or  principle  directing 
the  development  of  the  crystals. 

There  are  several  phenomena  in  the  production  of  flashed 
effects. 

If  a fire  flashed  buff  burning  brick  is  taken  from  the  hot 
kiln  in  the  final  stages  of  the  flashing  process,  it  does  not 
show  the  flashed  color  but  instead  has  the  grayish  color  of 
a simple  reduction.  Mr.  J.  Parker  B.  Fiske  in  an  early  re- 
port of  the  American  Ceramic  Society  shows  the  advancing 
depth  in  the  flashed  color  of  the  bricks  taken  from  the  kiln 
at  intervals  during  the  cooling  process.  Time  is  essential  to 
permit  the  growth  of  the  crystals. 

We  have  taken  deeply  flashed  bricks  and  reburned  them 
in  the  top  of  a continuous  kiln  where  oxidizing  conditions 
were  a maximum  and  when  the  bricks  were  taken  from  the 
kiln,  the  flashed  color  had  largely  disappeared  and  instead 
we  had  the  grayish  color  of  a reduced  surface.  The  explana- 
tion is  that  we  have  burned  out  the  carbon  and  without  its 
activity,  the  crystals  cannot  develop,  but  complete  oxidation 
is  difficult  and  there  remains  a trace  of  the  flashed  color. 

One  company  in  flashing  fire  clay  products  gets  an  unsatis- 
factory dark  color,  but  not  the  desired  russet  color. 

The  operation  in  the  factory  is  to  finish  the  burning  with 
clear  fires  and  as  strongly  oxidizing  condition  as  possible, 
and  in  this  way  the  dark  unsatisfactory  color  is  overcome, 
leaving  a light  russet  flash. 

Fire  clay  products  can  be  flashed  at  relatively  low  tem- 
peratures by  starting  the  flashing  treatment  early  in  the 


334 


BURNING  CLAY  WARES. 


burning  operation  and  alternating  with  oxidizing  conditions, 
but  it  is  our  observation  that  such  flashes  are  lustreless  and 
lack  the  life  and  brilliancy  of  the  properly  developed  flash. 
Why  we  do  not  know.  Our  mental  process  runs  to  the  theory 
that  the  true  crystal  flash  is  in  some  way  due  to  graphitic 
carbon  from  cracked  gases;  that  cracking,  of  methane  par- 
ticularly, occurs  at  higher  temperatures;  that  soot  developed 
in  the  furnace  and  at  low  temperatures  and  held  mechanically 
in  the  gases  may  be  absorbed  by  the  ware  but  does  not  enter 
into  crystal  development.  It  may  have  the  effect  of  darkening 
the  surface  of  the  ware  but  the  true  flash  development  is 
limited,  or  hindered,  and  its  brilliancy  is  dimmed. 

Smoke  as  we  understand  the  term  is  not  essential  to  the 
production  of  the  flashed  color.  We  have  gotten  an  intense 
flash  on  a light  buff  burning  ware  with  natural  gas  and 
throughout  the  burn  there  was  no  smoke  in  evidence. 

The  true  flash  comes  only  when  the  surface  of  the  ware  is 
exposed  to  the  flame.  If  the  ware  is  set  in  saggers,  or 
muffles,  however  porous  they  may  be,  thus  permitting  the 
passage  of  kiln  gases,  we  do  not  get  the  flash  because  the 
walls  of  the  saggar  or  muffle  collect  the  graphite. 

A hole  or  crack  in  the  muffle  wall  which  permits  tongues 
of  flame  to  impinge  on  the  surface  of  the  ware  will  produce 
a flash  wherever  the  flames  touches.  Bricks  closely  set  end 
to  end  may  have  deeply  flashed  faces  but  the  ends  will  not 
be  flashed  except  shrinkage  provides  an  opening,  and  if  this 
opening  is  small,  the  flash  will  enter  only  a short  distance 
and  the  remainder  of  the  end  of  the  brick  will  not  be  flashed. 

As  a rule, — invariably  so  far  as  we  know, — we  cannot  get 
the  deep  golden  flash  on  a stiff  mud  product  such  as  we  get 
on  a dry  pressed  product  which  explains  why  the  dry  pressed 
flashed  product  has  held  its  market  which  in  other  color 
effects  it  has  been  largely  displaced  by  the  stiff  mud  product. 

The  deep  flash  is  easily  obtained  on  the  salt  glazed  stiff 
mud  product. 

The  quantity  of  absorbed  carbon  seems  to  influence  the 
crystal  development  although  it  is  not  necessarily  a consti- 
tuent of  the  crystal.  We  have  shown  that  we  can  reduce 
the  depth  of  the  flash  by  oxidation,  and  it  follows  that  in- 
creased carbon  content  will  deepen  the  flash. 

If  this  be  true  we  have  an  explanation  of  the  difficulty 
in  flashing  a stiff  mud  product.  Compared  with  the  surface 
of  the  stiff  mud  product,  that  of  the  dry  pressed  product  is 
very  porous,  and  in  consequence  the  surface  exposure  within 
a given  area  is  much  greater.  Since  the  flash  is  a surface 
phenomenon  any  increase  in  the  depth  of  the  surface  will 
give  corresponding  increase  in  the  development  of  the  flash. 
The  viscous  surface  of  the  salt  glazed  ware  will  take  up  the 
carbon  and  continually  present  a fresh  surface  for  further 
accumulation. 

The  intention  of  the  above  discussion  is  not  so  much  an 
attempt  to  explain  the  cause  of  flashing  concerning  which 


BURNING  CLAY  WARES. 


335 


there  is  little  definite  knowledge  and  much  conjecture,  but 
instead  we  wished  to  present  the  phenomena  in  so  far  as 
we  know  them  in  order  that  a better  procedure  may  be  de- 
veloped in  the  production  of  these  color  effects. 

The  best  flashed  effects  are  produced  at  high  temperatures, 
— cone  3 to  cone  9,  preferably  the  higher  temperature.  Reduc- 
ing kiln  atmospheres  are  commonly  employed  but  in  this  re- 
spect we  have  a field  worthy  of  investigation.  Excessive 
reduction,  and  by  that  we  mean  an  air  supply  far  below  the 
theoretical  requirement  for  perfect  combustion,  may  produce 
a darker  color,  but  it  is  less  brilliant  than  a lighter  reduction. 
In  fact  one  may  produce  the  golden  flash  at  high  temperatures 
with  an  excess  of  air  in  the  combustion  gases.  Flame  contact 
with  the  surface  of  the  ware  is  necessary.  The  slower  the 
cooling,  the  deeper  the  color  of  the  flash,  due  to  the  greater 
crystal  development.  Unsatisfactory  color  effects,  due,  per- 
haps, to  excessive  reduction,  sometimes  may  be  corrected  by 
finishing  the  burning  under  so  called  oxidizing  conditions. 

Fire  flashing,  or  more  properly,  reduction  of  red  burning 
clays  is  a very  different  effect  from  the  flash  on  fire  clay 
products. 

In  the  red  burning  clays  the  brown  to  gun  metal  color 
is  throughout  the  mass  and  only  more  brilliant  on  the  surface 
in  consequence  of  more  fusion  or  glazing  effect.  Whether 
there  is  any  graphitic  absorption  and  crystal  development  or 
not  is  not  known  but  considering  the  behavior  of  fire  clays 
at  low  temperatures,  it  is  likely  there  is  very  little  true 
flashing  in  red  burning  clays  and  any  slight  true  flash  is 
mantled  by  the  deep  brown  to  black  color  of  the  reduced 
iron  in  the  clay  mass  and  its  combination  with  silica.  In 
limey  clays  we  get  a green  color  which  is  as  truly  a flash 
as  the  brown  and  blacks  in  red  burning  clays.  In  the  one 
the  green  color  is  due  to  the  development  of  lime-iron-silicate 
and  in  the  other  the  brown  is  due  to  iron-silicate. 

In  a red  burning  clay  under  reducing  kiln  conditions  the 
red  oxide  of  iron  is  reduced  to  the  ferrous  oxide  which  acts 
as  a flux  and  combines  with  silica,  likely  also  with  alumina, 
to  form  a black  iron  silicate,  or  iron  alumina  silicate  to 
which  the  dark  color  is  due.  If  the  reduction  is  slight  the 
amount  of  black  iron  silicate  developed  suffices  merely  to 
darken  the  natural  red  color  to  a brown,  becoming  darker  with 
increased  reduction  and  finally  attains  the  dark  gun  metal 
black  in  which  the  iron  silicate  predominates. 

The  fused  matrix  will  take  into  combination  or  solution 
other  minerals  which  give  color  effects  other  than  the  brown 
to  gun  metal  black,  and  thus  we  may  get  the  olive  greens, 
saffrons,  and  purples.  If  the  flashings  is  done  quickly  the  re- 
duction may  not  penetrate  to  the  center  of  the  mass  of  the 
ware  and  thus  in  bricks  faced  in  the  kiln,  we  get  the  dark 
edges  and  red  centers. 

The  widest  variation  in  color  effects  comes  from  clays 
that  scum  badly  and  irregularly.  The  olive  greens  are  likely 


836 


BURNING  CLAY  WARES. 


due  to  lime  content  in  consequence  of  the  reduction  of  the 
lime  sulphate  (scum)  and  combination  of  the  lime  with  the 
iron  silicate,  just  as  a limey  clay  produces  a green  color 
when  hard  burned.  Such  a green  color  will  be  on  the  surface 
only,  except  the  color  comes  from  sufficient  lime  distribu- 
tion throughout  the  clay  mass  to  develop  the  lime-iron  green. 

The  saffron  colors  we  have  attributed  to  sulphur  effects 
in  the  water-smoking  and  in  incomplete  oxidation.  We  have 
noted  the  influence  of  sulphur  in  producing  unusual  colors 
and  we  believe  that  some  of  the  peculiar  color  effects  in 
reduced  products  may  come  from  sulphur.  One  yard  formerly 
getting  a percentage  of  unusual  colors  including  saffrons  no 
longer  produces  them  since  the  kiln  bottoms,  setting,  and 
kiln  drafts  have  been  corrected  to  get  better  and  quicker 
burns. 


Copyrighted  1920  by  T.  A.  Randall  & Co. 


BURNING  CLAY  WARES. 


337 


EQUALIZATION  TABLES 

EQUALIZATION  tables  are  used  by  engineers  in  design- 
ing structures  involving  the  movement  of  fluids,  but 
their  use  is  not  limited  to  engineers.  The  farmer  and 
drain  tile  maker  should  use  them  in  determining  the  proper 
sizes  of  mains  and  laterals.  The  factory  superintendent  who 
plans  the  kilns,  dryers,  stacks,  hot  air  and  gas  systems,  should 
have  at  hand  the  tables  and  not  follow  rules  of  thumb,  assump- 
tions, guesses,  which  are  only  too  common  in  clay-working 
factory  construction,  and  often  the  cause  of  inefficient  opera- 
tion and  considerable  loss. 

The  following  tables  are  used  in  our  engineering  work,  but 
have  been  re-calculated  and  extended  for  this  publication. 

The  equalization  table,  appearing  on  opposite  page,  is  for 
circular  pipes  of  given  diameters. 

The  numbers  to  the  left  are  the  diameters  of  the  mains 
and  the  numbers  at  the  top  are  the  diameters  of  the  laterals. 
The  numbers  in  the  body  of  the  table  are  the  equivalent  num- 
ber of  laterals  in  each  main. 

For  example,  if  we  wish  to  use  six-inch  laterals  and  desire 
to  know  how  many  of  them  are  equivalent  to  a 20-inch  main, 
we  find  on  the  first  page  of  the  tables  in  the  column  headed 
by  6 opposite  the  number  20,  the  number  20.3,  which  is  the 
number  of  smaller  circles  equivalent  in  carrying  capacity  to 
the  larger  circle. 

Assume  that  we  have  a 30-inch  main  and  wish  to  know 
how  many  8-inch  pipes  may  be  taken  off  from  it.  In  the  tables 
opposite  30  in  the  column  to  the  left  and  under  the  column 
headed  8 we  find  28,  which  is  the  number  of  pipes  required. 

Again  we  have  32  pipes  9 inches  in  diameter  and  wish  to 
know  the  size  of  the  main.  We  drop  in  column  headed  by  9 


338 


BURNING  CLAY  WARES. 


to  the  number  32,  and  opposite  this  in  the  column  on  the  left 
we  find  that  the  main  should  be  36  inches  in  diameter. 

The  table  is  extended'  to  cover  48-inch  mains  and  36-inch 
laterals,  but  it  is  possible  to  get  the  data  for  any  sizes.  Sup- 
pose we  have  a 60-inch  main  and  wish  to  know  how  many 
36-inch  laterals  it  will  require  to  equal  it. 

Sixty  is  beyond  the  limit  of  the  table,  but  if  we  halve  it 
and  the  size  of  the  lateral,  the  number  of  pipes  is  the  same. 

Halving  the  two  numbers  gives  us  a 30-inch  main  and  an 
18-incli  lateral.  Opposite  30  and  under  18  we  find  the  number 
of  laterals  to  be  3 6.  Or  we  may  quarter  the  sizes  of  the  pipes, 
or  divide  them  by  any  number  to  get  sizes  within  the  limits 
of  the  table  and  the  results  will  be  practically  the  same. 

For  example,  if  we  quarter  the  above  given  main  and  lat- 
eral wTe  have  15  inches  for  the  main  and  9 inches  for  the  lateral 
and  from  the  table  we  find  that  3.6  of  the  latter  are  equal  to 
the  former. 

Conversely,  if  we  have  given  the  number  of  lateral  pipes 
and  the  number  exceeds  the  limit  of  the  table,  we  may  deter- 
mine the  size  of  the  main  pipe  for  half  the  size  of  the  laterals 
and  double  the  result,  as  for  instance  sixty  10-inch  laterals  to 
find  the  main.  We  look  in  column  headed  5 and  find  that  62 
pipes  (the  nearest  number  to  60)  require  a 26-inch  main,  and 
therefore  a 52-inch  main  will  be  the  proper  size  for  approxi- 
mately 60  pipes  10  inches  in  diameter. 

The  problem  is  the  same  for  large  mains,  as  for  example, 
assume  we  have  a 60-inch  main  and  wish  to  take  off  100  lat- 
erals, what  will  be  the  size  of  the  laterals?  Since  60  is  not  in 
the  table  we  will  take  30  and  we  find  that  100  is  between  154 
in  the  column  headed  by  4,  and  88  in  the  column  is  headed  by 
5,  but  it  is  nearer  the  latter  than  the  former.  Evidently  the 
size  of  the  lateral  is  between  9 and  10  and  the  size  used  will 
depend  upon  whether  one  wishes  the  carrying  capacity  of  the 
main  or  the  laterals  to  be  in  excess. 

If  one  desires  the  exact  size,  he  must  resort  to  the  formula 
which  is  the  ratio  of  the  square  roots  of  the  fifth  powers  of 
the  diameters  of  the  circles.  Where  the  numbers  are  small 
one  may  interpolate  in  the  tables  and  get  a close  approxima- 
tion of  the  exact  size,  but  interpolation  is  liable  to  wide  error 
where  the  numbers  are  high. 


BURNING  CLAY  WARES. 


339 


Tables  of  Rectangles  in  Equivalents  Circles 


HE  ducts  with  which  the  engineer  has  to  deal  are  quite 


as  often  rectangular  as  circular,  and  the  following  tables 


in  conjunction  with  the  Equalization  Tables  enables  one 
to  get  the  necessary  data  for  rectangles. 

The  numbers  across  the  top  and  in  the  left-hand  column 
of  each  table  are  the  respective  sides  of  the  rectangles.  The 
numbers  in  the  body  of  the  tables  are  the  diameters  of  equiva- 
lent circles. 

The  use  of  the  tables  is  best  shown  by  illustrations. 

Suppose  we  have  a square  stack  for  a down-draft  kiln  and 
wish  to  properly  proportion  the  kiln  floor  ducts,  and  also  to 
make  the  main  draft  flue  a rectangle  with  unequal  sides.  We 
will  assume  the  stack  to  be  36  inches  square.  The  main  draft 
flue  in  the  kiln  should  be  equivalent  in  size.  In  table  8,  oppo- 
site 36  on  the  left,  in  the  column  headed  by  36,  we  find  the 
diameter  of  the  equivalent  circle  to  be  39.7.  We  may  be  lim- 
ited in  the  depth  of  the  main  draft  flue,  or  the  arrangement 
may  limit  us  in  the  width.  Assume  that  we  may  make  the 
draft  flue  48  inches  deep,  what  should  be  the  other  dimension? 

In  table  6,  opposite  48,  in  the  left-hand  column,  we  find  39.3 
under  the  column  headed  by  27,  and  40.1  in  the  column  headed 
by  28.  Since  the  circle  equivalent  to  the  stack  is  39.7  inches 
in  diameter,  the  width  of  the  draft  flue  having  a depth  of  48 
inches  will  be  between  27  inches  and  28  inches. 

We  may  assume  a width  of  24  inches  for  the  draft  fine 
and  in  table  6 in  the  column  headed  by  24  we  find  39.7  to  be 
the  equivalent  circle  for  56  inches,  which  is  the  proper  depth 
for  the  flue  if  the  width  is  24  inches. 

Let  us  assume  that  there  are  twenty  kiln  floor  ducts  to  be 
drained  by  the  main  draft  flue  and  the  stack.  The  stack  and 


340 


BURNING  CLAY  WARES. 


draft  flue  equivalent  circle  is  39,7  inches,  as  shown  above.  We 
wish  to  find  the  the  proper  size  for  twenty  rectangular  floor 
ducts. 

We  turn  to  the  equalization  tables  and  find  that  a 40-inch 
circle  is  equivalent  to  twenty  12-inch  circles.  Twelve  inches 
then  is  the  diameter  of  the  equivalent  circle  for  the  ducts.  In 
building  these  ducts  one  or  the  other  dimension  will  be  lim- 
ited. For  instance,  we  may  wish  to  use  a 13-inch  floor  block 
and  the  width  of  the  duct  should  be  9 inches.  What  depth 
must  the  duct  be  to  equal  a 12-inch  circle? 

In  the  column  headed  by  9 in  the  rectangular  table  1 we  find 
11.9  to  be  the  nearest  circle  to  12  inches,  and  the  other  di- 
mension of  the  duct  corresponding  to  this  circle  is  13  inches. 
The  proper  size  for  the  kiln  floor  ducts  is  therefore  13  inches 
by  9 inches.  It  may  be  that  the  engineer  is  not  limited  in 
either  width  or  depth,  or  he  may  be  limiited  in  both  dimen- 
sions, and  wishes  to  get  a compromise  rectangle  which  will 
best  meet  the  limits.  The  equivalent  circle  is  12  inches.  He 
may  start  with  a square  which  he  will  find  to  be  11  inches  in 
table  3,  equivalent  to  a 12  1-inch  circle.  Starting  with  the  lat- 
ter circle  in  the  body  of  the  table  and  running  up  diagonally 
to  the  right  he  will  find  a 12-inch  circle  equivalent  to  a 10-incli 
by  12-inch  rectangle,  or  a 11.9  circle  is  equivalent  to  a 9-inch 
by  13-inch  rectangle,  and  it  is  also  equivalent  to  an  8-inch  by 
15-inch  rectangle,  or  a 12.1-incli  circle  is  equivalent  to  a 7-inch 
by  18-inch  rectangle.  He  may  also  work  diagonally  down  to 
the  left  and  pick  out  the  dimensions  which  best  suit  his  con- 
ditions. For  instance,  in  table  1,  in  the  last  column,  we  find 
that  a 12-inch  circle  is  equivalent  to  a 10-inch  by  12-inch  rec- 
tangle, or,  in  the  next  column;  a 11.9-inch  circle  is  equivalent 
to  a 9-inch  by  13-inch  rectangle,  and,  next,  to  an  8-inch  by  15- 
inch  rectangle.  Further  scanning  shows  that  the  approximate 
size  of  the  rectangle  may  be  7 inches  by  18  inches,  6 inches  by 
21  inches,  5 inches  by  27  inches,  or,  turning  to  table  2,  we 
find  the  rectangle  may  be  4 inches  by  36  or  37  inches,  3 inches 
by  55  inches.  He  may  wish  narrow  slots  2 inches  wide,  but 
finds  that  the  depth  exceeds  the  limits  of  the  table.  He  may 
double  the  dimension,  and  find  the  corresponding  other  di- 
mension and  doubling  this  get  approximately  the  depth  re- 
quired. In  table  2 for  a width  of  4 inches  the  depth  will  be 
36  inches  or  37  inches  and  the  proper  depth  for  a 2-inch  slot 
would  be  72  inches  or  74  inches.  In  such  extreme  dimensions 
there  is  always  likely  to  be  an  error  since  the  table  is  only 
worked  out  to  the  nearest  tenth. 


BURNING  CLAY  WARES. 


341 


It  usually  happens  that  the  tables  are  exceeded  only  in 
larger  tlues  and  the  error  in  such  instances  is  slight. 

The  tables  may  be  checked  up  by  taking  dimensions  within 
the  limits  and  doubling  and  halving  them.  For  example,  a 
duct  40  by  60,  as  may  be  seen  in  table  12,  has  an  equivalent 
circle  of  53.7  inches,  or  the  same  is  found  in  table  8.  A 20  by 
30  duct,  which  would  be  halving  the  dimensions  of  the  above 
larger  duct,  has  an  equivalent  circle  of  26.9,  as  shown  in  table 
5 or  in  table  3.  Doubling  this  equivalent  circle,  we  get  53.8, 
which  practically  checks  the  40  by  60  dimensions.  If  there- 
fore we  had  an  80-inch  by  90-inch  duct  we  would  look  up  the 
equivalent  circle  for  a 40  by  45  duct  which  we  find  in  table  8 
or  table  10  to  be  46.7,  and  the  equivalent  circle,  for  the  larger 
duct  will  be  double  this,  or  93.4. 

The  several  problems  are  as  follows : 

(1)  Given  a square  duct  or  a rectangular  one  of  fixed  di- 
mensions, we  wish  to  find  equivalent  ducts  having  different 
dimensions.  This  is  done,  as  has  been  shown,  by  finding  in 
the  tables  of  rectangles  the  equivalent  circle  for  the  duct 
given,  and  picking  out  equivalent  circles  diagonally  up  to  the 
right  or  down  to  the  left  and  taking  the  dimensions  corre- 
sponding. 

(2)  Given  a main  duct,  we  wish  to  find  the  proper  size 
of  a fixed  number  of  smaller  ducts.  This  problem  has  been 
fully  worked  out  above. 

(3)  As  a corollary  to  No.  2,  we  have  small  ducts  or  fixed 
dimensions  and  wish  to  know  how  many  of  them  can  be 
turned  into  a larger  duct  of  fixed  dimensions. 

We  first  find  the  equivalent  circle  of  the  small  duct  and 
also  that  of  the  large  duct  from  the  tables  of  rectangles  and 
from  the  equalization  table  determine  how  many  of  the  smaller 
circles  equal  the  larger  circle. 

(4)  We  have  given  the  dimensions  and  number  of  small 
ducts  and  wish  to  determine  the  size  of  the  main  duct. 

We  first  find  the  equivalent  circle  for  the  small  ducts  from 
the  table  of  rectangles  and  then  in  the  equalization  table  find 
the  proper  circle  for  the  number  of  smaller  circles,  and  fin- 
ally, assuming  one  dimension  of  the  main  duct,  we  find  in  the 
table  of  rectangles  the  other  dimension  corresponding  to  the 
equivalent  circle  as  found. 

Should  the  dimension  thus  found  be  too  large  or  too  small 
for  our  purpose,  we  can  change  it  by  assuming  a different  di- 


342 


BURNING  CLAY  WARES. 


mension  in  the  first  instance,  and  thus  we  may  pick  out  from 
the  table  of  rectangles,  dimensions,  corresponding  to  the 
equivalent  circle,  which  suit  our  purpose. 

Should  the  sizes  of  the  ducts  exceed  the  limits  of  the  tables, 
we  can  bring  them  within  the  limits  by  halving,  and  get  the 
correct  sizes  by  doubling  the  dimensions  thus  found,  and  this 
holds  true  in  any  proportionate  part. 


Table  1.  Table 


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CMt-CMt-rH©rHnO©COOOCM©©rHOOrHLO>©COb-©rHb-rHrHOOrHrHOO 

HHMClC0C0^^T)ik0  1OCDCPt-b-h-00G000CiCJodc>HHH(^(NCq 
COCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOrHrHrt<rt<THrHrHrHrH 

1(0 

o\ 

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©T-li-!CMCMOMcdcdrHrHnondnO©©t-l>t'-o6oOo6©©©©©©rHrHrH 

COCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOrHrHrH^rHrH 

rH 

Cl 

©nO©''tfOOCMt-rHnO©eOt-rHnOOOCM©©COt~©COl>'©COt-©CO©© 

©©©rHrHCMCMcdcdcOrHrtodnonod©©b-ldo6oOo6©>©©>C>dd© 

COCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOrH'tirHrH 

CO 

CM 

COOOCMb-rHnOdrHOOCMlO^COt-rHTtfOOrHlOOOCMnOOOiHnOOOTHrHt-© 

©©©©rHrHCMCMCMCOcdcdrtlrHnonOnO©©©t~b-t^o6oOo6©©©© 

CMCMCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOrfi 

Cl 

CM 

b-rHnO©©OOCM©©''tlt>rHnO©CM©©CO©©CO©©CMKOOOCMnOOOrH 

coddooOrHr-icMCMCMcdcocddd^jiOiodcoddt^t-rdooooood 

CMCMCMCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCO 

rH 

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©rH©COt~T-ino©CM©©COt>©rHt-T-HrHt-rHrHb-©CO©©CMnOOOrH 

GCa6aO©©d©©rHrHCMCMCMcdcOCOrHrH^»onOnO©©©>©b-ldl>o6 

CMCMCMCMCMCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCOCO 

rHCMCO^nO©b-O0©©rHCMCOrHnO©t~O0©©rH<MCOrHnO©b-O0©© 

C0C0C0C0C0C0C0C0C0rHrHTtfrHrH^H''frHrHrHL0n0n0n0n0n0n0n0IOU0© 

© 

rH 

rf 

o 

CO 

00 

CM 

rH 

nO 

IO 

LO 

HO 

rH 

CM 

rH 

© 

b- 

rH 

CM 

© 

CD 

CM 

© 

no 

rH 

t- 

CO 

© 

no 

© 

© 

b- 

05 

rH 

01 

CO 

IO 

© 

l> 

00 

© 

© 

rH 

CM 

CO 

CO 

rH 

no 

CD 

CD 

b- 

00 

00 

© 

© 

© 

tH 

rH 

CM 

CO 

rH 

rH 

rH 

rH 

rH 

rH 

tH 

rH 

CM 

<M 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

© 

CO 

CO 

CO 

CO 

CO 

00 

CO 

CM 

05 

rH 

CD 

o 

CM 

CM 

CM 

CM 

rH 

rH 

© 

b- 

no 

CO 

© 

b- 

rH 

rH 

00 

rH 

© 

© 

CM 

00 

rH 

© 

no 

CM 

rH 

t- 

05 

o 

CO 

JO 

© 

l> 

00 

© 

© 

tH 

rH 

CM 

CO 

rH 

no 

no 

© 

b- 

b- 

00 

© 

© 

© 

© 

rH 

rH 

CM 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

CM 

<M 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CO 

© 

CO 

CO 

CO 

CM 

rH 

b- 

CM 

rH 

GO 

05 

o 

05 

05 

oo 

no 

rH 

rH 

© 

CD 

CO 

© 

l- 

CO 

© 

no 

rH 

L- 

© 

00 

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© 

CM 

rH 

b- 

05 

o 

CO 

IO 

tH 

tH 

00 

© 

© 

rH 

CM 

CO 

CO 

rH 

no 

© 

© 

l> 

t- 

00 

© 

© 

© 

© 

rH 

rH 

rH 

rH 

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rH 

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rH 

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tH 

CM 

<N 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

© 

© 

CO 

CO 

b- 

rH 

05 

CO 

o 

CM 

no 

t- 

t- 

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10) 

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CM 

© 

l> 

no 

CM 

© 

© 

CM 

00 

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© 

© 

CM 

b- 

CO 

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£m 

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oo 

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CO 

to 

© 

IH 

00 

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© 

rH 

CM 

CM 

co 

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i£3 

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© 

b- 

00 

00 

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© 

© 

© 

rH 

rH 

rH 

rH 

tH 

rH 

rH 

rH 

rH 

rH 

CM 

<M 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

co 

CO 

CO 

(•(^1 

CO 

05 

00 

rH 

oo 

o 

CO 

rH 

rH 

CO 

CO 

CM 

© 

00 

CO 

CO 

rH 

t- 

rH 

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CO 

© 

no 

rH 

© 

CM 

b- 

CM 

b- 

CM  j 

TH 

CO 

00 

o 

rH 

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to 

© 

Jh 

00 

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© 

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CO 

rH 

no 

no 

© 

© 

b- 

00 

oo 

© 

© 

© 

© 

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rH 

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© 

no 

00 

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no 

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00 

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00 

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© 

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no 

no 

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© 

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rH 

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CM 

Ol 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

CM 

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no 

t- 

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00 

00 

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CO 

no 

CO 

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no 

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00 

no 

rH 

b» 

© 

© 

rH 

© 

no 

© 

no 

© 

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co 

00 

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CO 

no 

CO 

tH 

00 

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© 

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no 

no 

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b- 

b- 

00 

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© 

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rH 

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<M 

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CM 

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CM 

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CM 

CM 

CM 

CM 

CM 

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CO 

rH 

00 

CM 

rH 

nO 

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1C 

rH 

CO 

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© 

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rf 

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CO 

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00 

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CO 

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00 

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COnOCM©©CMCOCMCMrH©b-nOCM©©CO©nOTHt'.CMb-COOOCOOOCOOOCM 


CMrH©0O©rHCMCOrtfnO©©b*O0©©©rHrHCMCOCOrHrHliOnO©©l>t-O0 

rHrHrHrHrHrHrHrHrHrHrHCMCMCMCMCMCMCMCMCMCMCMCMCMCMCM 


ClCO©'*HOO©©©©00©COrHOOnOrHOOrti©©rHl>-CMI>CMb-CMt-iH© 


-H©GC©©CMCOCOrHnO©t-0000©©©rHCMCMCOCOrHrHnonO©©b-t~ 


rHCMC0THn3©l>00©©THCMC0rHlO©b-00©©rHCMC0rHn0©b-00©© 
| HHHrt  tH  rH  HHHHNN<N(NC^(NNW(N<MCO 


Table  7.  Table 


b- 

rH 

o 

CD 

CM 

oo 

rH 

05 

ID 

rH 

CD 

rH 

b- 

(M 

b- 

<M 

CM 

l> 

CM 

r> 

rH 

CD 

tH 

ID 

O 

rH 

05 

CO 

t- 

TP 

00 

CO 

05 

co 

§< 

o 

rH 

rH 

rH 

tH 

TH 

CO 

rH 

33 

ID 

rH 

ID 

rH 

3 

rH 

b- 

rH 

00 

rH 

00 

rH 

05 

rH 

05 

rH 

O 

ID 

O 

ID 

rH 

io 

rH 

iO 

CM 

ID 

CM 

ID 

CM 

D 

CO 

ID 

CO 

ID 

CM 

05 

ID 

rH 

b- 

CO 

00 

rH 

o 

ID 

O 

CD 

rH 

CD 

rH 

CD 

rH 

CD 

rH 

CD 

O 

ID 

O 

rH 

05 

CO 

00 

CM 

CD 

O 

CO 

on 

on 

05 

o 

o 

rH 

tH 

CM 

CO 

CO 

rH 

rH 

ID 

ID 

CD 

CD 

b- 

b- 

00 

00 

05 

05 

O 

O 

O 

rH 

rH 

CM 

CM 

CO 

CO 

co 

CO 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

ID 

ID 

ID 

\o 

IO 

D 

ID 

ID 

00 

rH 

O 

CD 

CM 

CO 

05 

rH 

05 

ID 

O 

ID 

O 

ID 

O 

D 

O 

rH 

05 

rH 

00 

CO 

b- 

CM 

CD 

rH 

ID 

05 

CO 

eo 

b- 

00 

05 

05 

o 

o 

tH 

rH 

(M 

<N 

CO 

rH 

rH 

ID 

ID 

CD 

CD 

b- 

b- 

b- 

00 

00 

05 

05 

O 

O 

rH 

rH 

rH 

CM 

CO 

CO 

CO 

CO 

rH 

rH 

rH 

rH 

rH 

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rH 

rH' 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

ID 

ID 

iO 

IO 

IO 

ID 

co 

05 

ID 

tH 

CD 

CM 

00 

CO 

00 

rH 

05 

rH 

05 

rH 

05 

rH 

05 

CO 

00 

CO 

t- 

CM 

CD 

rH 

ID 

05 

rH 

00 

CM 

CD 

CO 

b- 

b- 

on 

05 

05 

o 

o 

rH 

rH 

CM 

cm 

CO 

CO 

rH 

rH 

ID 

ID 

CD 

cp 

b- 

b- 

00 

00 

05 

05 

05 

O 

o 

rH 

rH 

CO 

CO 

CO 

CO 

CO 

rH 

rH 

rH 

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rH 

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rH 

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rH 

rH 

rH 

ID 

ID 

IQ 

IO 

00 

rH 

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ID 

rH 

b- 

CM 

o 

CO 

00 

CO 

00 

CO 

00 

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00 

CM 

l> 

tH 

CD 

rH 

ID 

05 

rH 

00 

CM 

CD 

O 

rH 

00 

CO 

CD 

1- 

on 

00 

05 

05 

o 

o 

rH 

rH 

CM 

01 

CO 

CO 

rH 

ID 

ID 

CD 

CD 

b- 

b- 

b- 

OO 

00 

05 

05 

o 

O 

o 

co 

CO 

co 

CO 

CO 

CO 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

ID 

ID 

ID 

CO 

oo 

rH 

O 

CD 

rH 

CD 

CM 

t- 

CM 

t- 

CM 

t> 

<N 

CD 

rH 

CD 

O 

ID 

05 

rH 

00 

CM 

t> 

rH 

ID 

05 

CO 

t- 

tH 

CO 

CD 

CD 

b- 

00 

00 

05 

05 

o 

o 

tH 

rH 

<M 

CM 

CO 

CO 

rH 

rH 

ID 

ID 

ID 

CD 

CD 

b- 

b- 

00 

CO 

00 

05 

05 

o 

CO 

CO 

co 

co 

CO 

CO 

CO 

rH 

rH 

tH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

»D 

00 

CO 

05 

rH 

o 

ID 

tH 

CD 

tH 

CD 

rH 

CD 

rH 

ID 

O 

ID 

05 

rH 

00 

CM 

t- 

tH 

ID 

05 

rH 

00 

CM 

CD 

05 

CO 

s 

»o 

CD 

CD 

b- 

00 

00 

05 

05 

o 

O 

rH 

rH 

CM 

CM 

CO 

CO 

CO 

rH 

rH 

ID 

ID 

CD 

CD 

CD 

b- 

b- 

00 

00 

00 

05 

CO 

co 

CO 

CO 

CO 

co 

CO 

CO 

rH 

rH 

TP 

Th 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

CM 

00 

co 

05 

rH 

o 

ID 

o 

ID 

O 

id 

o 

rH 

05 

rH 

00 

CO 

t- 

tH 

CD 

O 

rH 

oo 

CM 

CD 

O 

rH 

00 

CM 

CD 

v») 

CO 

id 

ID 

CD 

CD 

b- 

00 

oo 

05 

05 

o 

o 

rH 

rH 

rH 

<M 

CM 

co 

CO 

rH 

rH 

»D 

ID 

ID 

CD 

CD 

b- 

b- 

b- 

00 

GO 

CO 

co 

CO 

CO 

CO 

CO 

CO 

CO 

CO 

rH 

rH 

th 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

<*\1 

b- 

<M 

00 

CO 

00 

rH 

05 

rH 

05 

rH 

00 

CO 

00 

(M 

t- 

tH 

CD 

O 

rH 

05 

co 

rH 

ID 

05 

CO 

O 

rH 

00 

UN 

CO 

rH 

ID 

ID 

CD 

CD 

t- 

b- 

00 

00 

05 

05 

o 

o 

rH 

rH 

CM 

CM 

CO 

CO 

CO 

rH 

ID 

ID 

ID 

CD 

CD 

t- 

b- 

b- 

CO 

co 

CO 

CO 

CO 

CO 

co 

co 

CO 

CO 

CO 

rH 

rH 

tH 

rH 

rH 

rH 

rH 

rH 

rH 

tH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

tH 

r- 

CM 

00 

CO 

00 

CO 

00 

<M 

t- 

<M 

b- 

rH 

CD 

o 

rH 

05 

co 

l- 

rH 

ID 

05 

CO 

rH 

»D 

05 

CO 

CD 

O 

CO 

rH 

rH 

ID 

ID 

CD 

CD 

b- 

b- 

00 

00 

05 

05 

o 

O 

rH 

tH 

rH 

CM 

CM 

CO 

CO 

CO 

rH  rH 

ID 

ID 

D 

CD 

CD 

b- 

CO 

co 

CO 

CO 

CO 

CO 

co 

co 

CO 

CO 

co 

co 

rH 

rH 

tH 

tH 

rH 

rH 

rH 

rH 

rH 

rH  rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

CM 

CO 

rf 

ID 

CD 

I— 

00 

05 

o 

rH 

(N 

CO 

rH 

ID 

CD 

t- 

00 

05 

O 

rH 

CM 

CO 

rH 

ID 

CD 

b- 

00 

05 

O 

CO 

CO 

CO 

CO 

co 

CO 

CO 

CO 

CO 

rH 

tH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

rH 

ID 

iO 

ID 

ID 

ID 

ID 

ID 

1D 

ID 

ID 

CD 

O 

rH 

LDC0lDIDCM00CMCD05rHCMC0rHrH^HC0CMrHO00CDrHCM05CDC0Ob-rHTH 

lD00OCMrH»Dt-00  05THCMC0rHlDCDb-00  05OOrHCMC0C0rHlDCDCDb-00 
tHtHtHt— IiHi—ItHCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCOCOCOCO 

05 

CO 

IDCM^COHCDOrHCDOOOHHHHOOSOOCD^CMOQOlOMDCPCOO© 

!DOOOCMrH>Dt-o6o50CMCO'^VDCDb^b-o6o50rHCMCMCOrHrHlDCDb-b- 
tHtHtHtHtHt— IrHCMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCOCOCO 

00 

CO 

rHrHCOCM05rHOOCMrHCDt-OOOOOOOOb-CDrHCOrH05CDrHrHOOlDCMOOlDrH 
lD00OCMC0iDCD00  05OrHDlC0rHlDCDb-00  05OOrHCMC0C0^»DlDCDb” 

hhhhhhhcmcmcmcmcmcmcmcmcmcmcococococococococococo 

b- 

co 

C0OCMrH00C0CDOCM^»DlD»DlDrHC0CMrH05b-lDCMOb-rHrHb-rHOb- 

IDOOOCMCOlDCDGOOSOr-lDlCOrHlDCDt^OOOOOSOrHCMCMCOrHrHlDCDCD 

tHtHtHtHtHtHtHDICMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCOCO 

CD 

CO 

C0OrH05CDrHlD00OrHCMCMCMCMrHO05b-lDC0rH00lDCM05CDC005CDCM 

lD00OrHC01DCDI>05OTHCMC0rHiDCDCDb-00  05OOrHCMCMC0rHrHlDCD 
rHr-iiHiHT— ItHt— ICMCMD4CMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCOCO 

ID 

CO 

CM05O500rH05C0VDI>00O505O5C!500CDlOC0r-!O5b-rHrH00lDTH00rH!-lb- 

lDb“05i— icOrHCDb-000501— ICMC0rHlDCDb"00G005Oi— It— ICMCOCOrHlDlD 
tHtH^tHtHtHtHDICMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCO 

rH 

CO 

CMO0QOb-CMb*T— IC0lDCDb-b-CDCDlDC0CMO00lDC0Ob*rHOb-C005CDCM 

lDb“05rHC0rHCDb”0005Or-lCMC0'rHlDCDb-b-00  05OOr-i0'lDlC0C0rHlD 
tHt— ItHt-Ht— lr-lr-iCMCMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCO 

CO 

CO 

iHt-t-lDTHJDOOTHCOCOrHrHCOCMrHOOOCDrHrH051DCM05CDCMOOrHrHCD 

lDb-05rHC0rH»Dl>00  05OrHCMC0rH»DlDCDt-00  00  05OOTHCMCMC0rHrH 
THrHrHiHrHiHrHCMCMCMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCO 

CM 

CO 

OCDCDrH05C0CD05OHHTHG05  00CDrHCMOb-rHrH00»D'Hb-C005lDTH 

lDl>05rHCMrti»DCDo6o50THCMCMCOrHK5CDb-b^o6o505C)rHTHCMCMCOrH 

rHTHiHTHTHrHTHDlCMO^CMCMCMDlCMCMCMCMCMDlCOCOCOCOCOCOCO 

rH 

CO 

OiDlDCMb-OrHb-GOOOQOOOb-CDlDCOrHOOCDCOOb-COOCDOlOOrHOCD 

l0b*05HCJr)ilDCDb-0005OH(M"c0rHL0  1CCDb*00  00DOOHHCMC0C0 
tHtHtHtHtHtH-HtHCMCMCMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCO 

iHCMCOrH10COb-00050T-ICMCOrHlDCDb»a0050T-ICMCOrH10CDb-G0050 
tH  tHtHtHt-ItHtHtHtHtHCMCMCMCMCMCMDICMCMCMCO 

Table  9.  Table  10. 


kD 

rH©©d©©CO©©dOOrH©©dOOrH©kDrH©Clt>db«COOOCOOOCO 

COCOrHkDkD©b^OO©©©rHrHoidCOrHrHkDkD©©>b-b-OOo6©©©* 

rHrHrHrHrHrHrHrHrH''HrHkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkD© 

3 

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rHrHrHrH'<HrHrHrHrHrHrHrHlDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkD 

00 

rH 

COOt-'tOt-COOCOINX^OCOlMl-MOJ'HOWOlOOCOH^HWO 

dCOCOrHkDkD©>b^b^o6o6©©©rHrHdoicOrHrHkDkD©©t~b-0000© 

rHrHrHrHrH-'HrHrHrHrHrHrHkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDkDlDkD 

b- 

rH 

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kD 

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CO 

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Table  11.  Table  12. 


s 

©aOCOCOrHOOCOCOOt-THrHOOtOTHOO'^OL'-COCilOr-lt'-COGO^OtOT-l 

t-tr—  GOOiOOrHOJCOCO^lOlOCOt—  tr— 0Oci<d©©TH  OQoicOCO'^lOlOCO 
’'tf'^TtiTtliOlOlOlOtOtOtOtOlOlOlOlOlOtOtOCOCOCOCOCOCDCDCOCDCOCD 

a 

iO 

CD^<MOib-r^(M05CDCOOl>COOb-C005IO(MOOTtHOCD(Mt>COOi'<tlOiC 

<^^C»^CiOiHrH<^C0TH^10CDCDt^t>06cia}drHrH0idc0’c0TH»OlO 

''tfThiTtfTFTtHlOtOlOlOlOtOlOlOlOlOlOtOlOlOlOCOCOCOCDCDCDCOCDCOCO 

00 
l o 

CO©OOCDCO©OOlO<MC5CD<MC5tO<NOO^r-lt-COC}lOr-ICO<NOOCO©TtH© 

^b-^oooioodoioqcO'rHr^iocDCDt^oooodoioTHTHocidcoco^io 

•'tf^H'^^-^liOtOlOtOtOlOlOlOlOlOlOtOlOtOtOlOCOCDCOCOCDCDCOCOCO 

tr- 

io 

05t-^<MCDCD^rHOO'^iHOO'^THI>C005CD(MOO'^lCilOrHb-(MOOCOOiTH 

OCDh-OOOOC50rHrH(MCOCO'«^lodcDCDr>OOOodoiorHrH(N(MCOCOTH 

^TfHTjH-tfTtiTHlOlOlOtOlOtOlOlOlOlOlOlOlOlOlOlOCOCOCOCOCOCOCOCD 

CD 

O 

10COOOOlO(M05CDCOOt-COOCD<MOOlOHb-COOOT^OCDHI><MOOCOOO 

lOCDb-t-OODiOiOTHddeOTtHT^lOlOCDt^t^OOOOoiddrHrHdoqcOCO* 

■^•^■^•^'^TtH'^OlOlOlOlOlOlOlOLDlOlOlOlOlO-lOCOCOCDCDCDCDCDCD 

to 

IO 

r-l©CO^r-IOOlO<MCilO<NOOlOTHt-CO©'CO<Nt-COC5lO©COr-ll><Mt-CO 

^OCDb-OOOoddOrHC^dcOTH^lOCDCDt>r>o6o6dddrHT-idoicO 

'^'^■^■^TiHTtlTjHlOlOlOlOlOlOlOiOlOlOlOiOlOlOlOlOCDCDCDCDCDCDCD 

to 

t-10<MC5b-TtHTHt-'tfr-lt-Tfi©CDCOC5lO©CO<MOO''fCilO©COTHCO<Mt- 

'^lOCDCDt>00<dd©rHTHoicOCO'rH''tflOCDCDt~t^o6o6cS©dr-lTHC'l<N 

'tf'^^''frl^''*lTtl''^tOl010tOtOlOt010tOtOlO)010lOlOlOCDCOCOCOCOCO 

CO 

IO 

eOiHOOlO<NC5COCO©CDCOC5lOTHOOCOC:iOTHt-eOOOT*lO}10©tOr-ICOr-! 

^lOlOCDt^I>Ood©©rHrHoicOCO^THlOCDCDt-t-o6ood©©iHr-lC<i 

'^■'frl'^TtlTtH^tiTti^lOtOtOlOlOlOlOlOlOiOlOlOlOlOlOtOlOCOCDCOCOCD 

IO 

Dit-Tt<THOOtO<MOOlOr-IOO'<tf©COCOOOTt<©CO<Mt-COOO''tfCiTtHC5lO©tO 

COT^lO©CDb-o6o6©©dr4d(MCOCO^l010CDCDb-b-o6oO©©©rHr-l 

'^'^^'^TjHrtlTjHTti'^iQioiOlOlOlOlOlOlOOOlOOlOiOlOlOlOCD^CD 

iH 

to 

lOCOOt-'^T-lt-TtlOt-eoasiOr-ltr-COCSlOrHCO'Mt-COOOCOOO'^ClTtHCS 

coT^ioiocDb-b^o6doi©©rHoic\icocoTH»oiocDCDb^i>ooo6oioi©© 

'^^^^^•^^'H^^iOlOlOlOOlOLOWmiOlOWlOlOlOlOLOOCpCD 

s 

rH<NCOTfnoCOb-OOC5©r-l<NCOTt<10COt-OOCl©TH(MCOTHlOCOt-aOCi© 

COCOCOCOCOCOCOCOCOTtl'^lT^TtlTfl'^^'^TtiTtHlOlOOlOlOiDlOlOOlOCD 

lOOOlOOOOOb-lOlHb-r-ltOOOWTHCOt-CiOr-liHrHTHOOCiOOb-tOCOlN 

CD05<M'^CDOO©(MCO»OCDt-oi©rHdcO»OCD'l>-o6ci©©THOieO'^lOCD 

iHrHrHiH(M<M(M(M(N(M<NCOCOCOCOeOCOCOCOeO^Tt<^TtlTtlTtHTtlTfl 

a 

IO 

THb-''fCDt-C0C0D51Oa>C0CD©<NTtttOCDt-00  00  00  00t-CDC0''tlC0<M©00 
CD©C<I^©00©r-lc0TtHCDt^©©rH(NC0Tfl»OCDb-00d©THdc0^1O)O 

THrHr-lrHC4(M(MCq(M<M!MCOCOCOCOCOCOCOCOCOCO'^'^TtHT}HTtlrtlTfl 

00 

to 

^b-COlOCD^(MOOCOOOiHrtHt-©rHCO^TH»Ol010lOTt<CO(MrH©OOCDW 

CD00C^^cD00OrHC0^C0r>00©TH<NC0^1OCDb-06c5©rH0ic0C0Tt<lO 

rHrHrHrH(MC^iM(M<N(M<M(MC0C0C0C0C0C0C0C0C0'^'^TtlTti'>^TtlT^ 

t- 

to 

■^CD<M^lOCO©CDr-ICO©<MiOt-C5©r-l(M<N<M<M<NT-l©C500COlOCOTH 

CDD5(M^CDOO©rHCO'^COt-o6oi©D'icOr^lOCDI>00©©©TH(MCO^»0 

THr-lrHr-l<MC\IC^(^(MOqC^C'ICOCOCOCOCOCOCOCOCOTtH'^i^T^TtlrJHT^ 

CD 

to 

C0lOTHC0C0<MC5lO©TH00©C0lOt-00C5C5C5CiC500  00t-CDTt<C0r-ICit- 
CDOi(MTt^©OOoilHeO'^iiO^^OOoi©r^(^^COr^^lOCD^^OO©©^H(^^eOCO,  Tf 

rHiHrHrHrHCq<M!M(M(MCa<MCOCOCOCOCOCOeOCOCOCO^TtlTjiTtlT^Tti 

IO 

to 

COlO©(MCO©t-COOO<NCOOOrH<NTt<tOCOCDI>COCOl010CO<MrHC5t-CDCO 

CD05<M-^CDOO©r-lcqrfHiOCDOoddrHDlCOr*HlOCDI>o6©©iHrH(NCO'^ 

tHtHtH'MtH<M(MC\|(M(MOJMCOCOCOCOCOCOCOCOCOCO'^^'>^'^'^^ 

T* 

to 

<M^©TH<M©CDrHCD©COCDOO©(N(NCOTtfTlHCOCO(NTH©CSI>CDT^(N© 

CD©<MT}HCDt-ciTHoi^lOCDt^oi©rHDlCOTt<»OCDl>o6©©©TH(MCOTH 

THTHiHrHrH(N(M<N<M<MCqDJCOCOCOCOCOCOCOCOCOCOCOTt('<^TtlTt<Tt1 

CO 

to 

OlC0©O©00r^©Tt(00rHrflCDt-©©r-lTHrH©©©00b-CD'^Dl©00CD 

CDdrH^CDb-©rHOlC01OCDl^06©rHC<Jc0'^llLOCDCDb-000i©rHC^(NC0 

THrHrHrHrH04(MC-l(M<MC^(M(NCOCOCOCOCOCOCOCOCOCO^TlHTtiTtlT^ 

Ol 

IO 

iH<OtOO©>CDCDCOOOCOCDC5<MTtilOt-t-OOOOQOt-tr-COtOCO<N©OOCD''tl(M 

CD©THC010l>05©<MCOTtlCDI>00©©T-l(MCOTtilO©b-0005©©rH(NCO 

THrHrHTHrH(M<N<MCSJ(M(M<MOqCOCOOOCOCOCOCOCOCOCO^TtHTtH'^Tfi 

rH 

o 

O<Nt-O>t-lOr-lt-r-l''fit-©0JC0TH,''tilOlOlO''tiTHC0<M©C5t>tOe0©00 

CD©THCOtOt-®>©<NCOTticOb-OOC}©r-l<MCO''+llOCOt-OOOOCi©r-l<N<N 

THiHrHiHrH(MCl(MC^<M(N<M(MCOCOCOCOCOCOCOCOCOCOCO'^^MH'^ 

IrHC^CO^^OCDb-OOOSOTHiMCO'^lOCDb-OOOSOrHC^COTHlOCDb-OOOlO 
| HHHHHHHHHHCIMOOMOICICIWOKO 

GETTY  RESEARCH  INSTITUTE 


